diff --git a/application/src/main/java/org/opentripplanner/updater/trip/AddedStopTime.java b/application/src/main/java/org/opentripplanner/updater/trip/AddedStopTime.java
index 654c420da62..58a87c434e2 100644
--- a/application/src/main/java/org/opentripplanner/updater/trip/AddedStopTime.java
+++ b/application/src/main/java/org/opentripplanner/updater/trip/AddedStopTime.java
@@ -2,7 +2,9 @@
 
 import com.google.transit.realtime.GtfsRealtime;
 import de.mfdz.MfdzRealtimeExtensions;
-import java.util.Objects;
+import java.util.Optional;
+import java.util.OptionalInt;
+import java.util.OptionalLong;
 import javax.annotation.Nullable;
 import org.opentripplanner.gtfs.mapping.PickDropMapper;
 import org.opentripplanner.model.PickDrop;
@@ -13,49 +15,125 @@
  */
 final class AddedStopTime {
 
-  @Nullable
-  private final PickDrop pickup;
+  private final GtfsRealtime.TripUpdate.StopTimeUpdate stopTimeUpdate;
 
-  @Nullable
-  private final PickDrop dropOff;
-
-  public static final PickDrop DEFAULT_PICK_DROP = PickDrop.SCHEDULED;
-
-  AddedStopTime(@Nullable PickDrop pickup, @Nullable PickDrop dropOff) {
-    this.pickup = pickup;
-    this.dropOff = dropOff;
+  AddedStopTime(GtfsRealtime.TripUpdate.StopTimeUpdate stopTimeUpdate) {
+    this.stopTimeUpdate = stopTimeUpdate;
   }
 
   PickDrop pickup() {
-    return Objects.requireNonNullElse(pickup, DEFAULT_PICK_DROP);
+    return getPickDrop(
+      getStopTimeProperties()
+        .map(properties -> properties.hasPickupType() ? properties.getPickupType() : null)
+        .orElse(null),
+      getStopTimePropertiesExtension()
+        .map(properties -> properties.hasPickupType() ? properties.getPickupType() : null)
+        .orElse(null)
+    );
   }
 
   PickDrop dropOff() {
-    return Objects.requireNonNullElse(dropOff, DEFAULT_PICK_DROP);
-  }
-
-  static AddedStopTime ofStopTime(GtfsRealtime.TripUpdate.StopTimeUpdate props) {
-    if (props.getStopTimeProperties().hasExtension(MfdzRealtimeExtensions.stopTimeProperties)) {
-      var ext = props
-        .getStopTimeProperties()
-        .getExtension(MfdzRealtimeExtensions.stopTimeProperties);
-      var pickup = ext.getPickupType();
-      var dropOff = ext.getDropoffType();
-      var dropOffType = PickDropMapper.map(dropOff.getNumber());
-      var pickupType = PickDropMapper.map(pickup.getNumber());
-      return new AddedStopTime(pickupType, dropOffType);
-    } else {
-      var pickDrop = toPickDrop(props.getScheduleRelationship());
-      return new AddedStopTime(pickDrop, pickDrop);
-    }
+    return getPickDrop(
+      getStopTimeProperties()
+        .map(properties -> properties.hasDropOffType() ? properties.getDropOffType() : null)
+        .orElse(null),
+      getStopTimePropertiesExtension()
+        .map(properties -> properties.hasDropoffType() ? properties.getDropoffType() : null)
+        .orElse(null)
+    );
   }
 
-  private static PickDrop toPickDrop(
-    GtfsRealtime.TripUpdate.StopTimeUpdate.ScheduleRelationship scheduleRelationship
+  private PickDrop getPickDrop(
+    @Nullable GtfsRealtime.TripUpdate.StopTimeUpdate.StopTimeProperties.DropOffPickupType dropOffPickupType,
+    @Nullable MfdzRealtimeExtensions.StopTimePropertiesExtension.DropOffPickupType extensionDropOffPickup
   ) {
-    return switch (scheduleRelationship) {
-      case SCHEDULED, NO_DATA, UNSCHEDULED -> PickDrop.SCHEDULED;
-      case SKIPPED -> PickDrop.CANCELLED;
-    };
+    if (isSkipped()) {
+      return PickDrop.CANCELLED;
+    }
+
+    if (dropOffPickupType != null) {
+      return PickDropMapper.map(dropOffPickupType.getNumber());
+    }
+
+    if (extensionDropOffPickup != null) {
+      return PickDropMapper.map(extensionDropOffPickup.getNumber());
+    }
+
+    return PickDrop.SCHEDULED;
+  }
+
+  private Optional<GtfsRealtime.TripUpdate.StopTimeUpdate.StopTimeProperties> getStopTimeProperties() {
+    return stopTimeUpdate.hasStopTimeProperties()
+      ? Optional.of(stopTimeUpdate.getStopTimeProperties())
+      : Optional.empty();
+  }
+
+  private Optional<MfdzRealtimeExtensions.StopTimePropertiesExtension> getStopTimePropertiesExtension() {
+    return getStopTimeProperties()
+      .map(stopTimeProperties ->
+        stopTimeProperties.hasExtension(MfdzRealtimeExtensions.stopTimeProperties)
+          ? stopTimeProperties.getExtension(MfdzRealtimeExtensions.stopTimeProperties)
+          : null
+      );
+  }
+
+  OptionalLong arrivalTime() {
+    return stopTimeUpdate.hasArrival()
+      ? getTime(stopTimeUpdate.getArrival())
+      : OptionalLong.empty();
+  }
+
+  OptionalLong departureTime() {
+    return stopTimeUpdate.hasDeparture()
+      ? getTime(stopTimeUpdate.getDeparture())
+      : OptionalLong.empty();
+  }
+
+  private OptionalLong getTime(GtfsRealtime.TripUpdate.StopTimeEvent stopTimeEvent) {
+    return stopTimeEvent.hasTime()
+      ? OptionalLong.of(stopTimeEvent.getTime())
+      : OptionalLong.empty();
+  }
+
+  int arrivalDelay() {
+    return stopTimeUpdate.hasArrival() ? getDelay(stopTimeUpdate.getArrival()) : 0;
+  }
+
+  int departureDelay() {
+    return stopTimeUpdate.hasDeparture() ? getDelay(stopTimeUpdate.getDeparture()) : 0;
+  }
+
+  private int getDelay(GtfsRealtime.TripUpdate.StopTimeEvent stopTimeEvent) {
+    return stopTimeEvent.hasDelay()
+      ? stopTimeEvent.getDelay()
+      : stopTimeEvent.hasScheduledTime()
+        ? (int) (stopTimeEvent.getTime() - stopTimeEvent.getScheduledTime())
+        : 0;
+  }
+
+  boolean isSkipped() {
+    return (
+      stopTimeUpdate.getScheduleRelationship() ==
+      GtfsRealtime.TripUpdate.StopTimeUpdate.ScheduleRelationship.SKIPPED
+    );
+  }
+
+  OptionalInt stopSequence() {
+    return stopTimeUpdate.hasStopSequence()
+      ? OptionalInt.of(stopTimeUpdate.getStopSequence())
+      : OptionalInt.empty();
+  }
+
+  Optional<String> stopId() {
+    return stopTimeUpdate.hasStopId() ? Optional.of(stopTimeUpdate.getStopId()) : Optional.empty();
+  }
+
+  Optional<String> stopHeadsign() {
+    return (
+        stopTimeUpdate.hasStopTimeProperties() &&
+        stopTimeUpdate.getStopTimeProperties().hasStopHeadsign()
+      )
+      ? Optional.of(stopTimeUpdate.getStopTimeProperties().getStopHeadsign())
+      : Optional.empty();
   }
 }
diff --git a/application/src/main/java/org/opentripplanner/updater/trip/TimetableSnapshotSource.java b/application/src/main/java/org/opentripplanner/updater/trip/TimetableSnapshotSource.java
index e3ec690237e..2bf523628d7 100644
--- a/application/src/main/java/org/opentripplanner/updater/trip/TimetableSnapshotSource.java
+++ b/application/src/main/java/org/opentripplanner/updater/trip/TimetableSnapshotSource.java
@@ -249,7 +249,7 @@ public UpdateResult applyTripUpdates(
               serviceDate,
               backwardsDelayPropagationType
             );
-            case ADDED -> validateAndHandleAddedTrip(
+            case NEW, ADDED -> validateAndHandleAddedTrip(
               tripUpdate,
               tripDescriptor,
               tripId,
@@ -303,7 +303,7 @@ public UpdateResult applyTripUpdates(
   /**
    * Remove previous realtime updates for this trip. This is necessary to avoid previous stop
    * pattern modifications from persisting. If a trip was previously added with the
-   * ScheduleRelationship ADDED and is now cancelled or deleted, we still want to keep the realtime
+   * ScheduleRelationship NEW and is now cancelled or deleted, we still want to keep the realtime
    * added trip pattern.
    */
   private void purgePatternModifications(
@@ -324,7 +324,7 @@ private void purgePatternModifications(
     ) {
       // Remove previous realtime updates for this trip. This is necessary to avoid previous
       // stop pattern modifications from persisting. If a trip was previously added with the ScheduleRelationship
-      // ADDED and is now cancelled or deleted, we still want to keep the realtime added trip pattern.
+      // NEW and is now cancelled or deleted, we still want to keep the realtime added trip pattern.
       this.snapshotManager.revertTripToScheduledTripPattern(tripId, serviceDate);
     }
   }
@@ -466,7 +466,7 @@ private Result<UpdateSuccess, UpdateError> handleScheduledTrip(
   }
 
   /**
-   * Validate and handle GTFS-RT TripUpdate message containing an ADDED trip.
+   * Validate and handle GTFS-RT TripUpdate message containing an NEW trip.
    *
    * @param tripUpdate     GTFS-RT TripUpdate message
    * @param tripDescriptor GTFS-RT TripDescriptor
@@ -490,20 +490,14 @@ private Result<UpdateSuccess, UpdateError> validateAndHandleAddedTrip(
     final Trip trip = transitEditorService.getScheduledTrip(tripId);
 
     if (trip != null) {
-      // TODO: should we support this and add a new instantiation of this trip (making it
-      // frequency based)?
-      debug(tripId, serviceDate, "Graph already contains trip id of ADDED trip, skipping.");
+      debug(tripId, serviceDate, "Graph already contains trip id of NEW trip, skipping.");
       return UpdateError.result(tripId, TRIP_ALREADY_EXISTS);
     }
 
     // Check whether a start date exists
     if (!tripDescriptor.hasStartDate()) {
       // TODO: should we support this and apply update to all days?
-      debug(
-        tripId,
-        serviceDate,
-        "ADDED trip doesn't have a start date in TripDescriptor, skipping."
-      );
+      debug(tripId, serviceDate, "NEW trip doesn't have a start date in TripDescriptor, skipping.");
       return UpdateError.result(tripId, NO_START_DATE);
     }
 
@@ -521,7 +515,7 @@ private Result<UpdateSuccess, UpdateError> validateAndHandleAddedTrip(
 
     // check if after filtering the stops we still have at least 2
     if (stopTimeUpdates.size() < 2) {
-      debug(tripId, serviceDate, "ADDED trip has fewer than two known stops, skipping.");
+      debug(tripId, serviceDate, "NEW trip has fewer than two known stops, skipping.");
       return UpdateError.result(tripId, TOO_FEW_STOPS);
     }
 
@@ -557,7 +551,7 @@ private List<StopTimeUpdate> removeUnknownStops(
           debug(
             tripId,
             serviceDate,
-            "Stop '{}' not found in graph. Removing from ADDED trip.",
+            "Stop '{}' not found in graph. Removing from NEW trip.",
             st.getStopId()
           );
         }
@@ -567,7 +561,7 @@ private List<StopTimeUpdate> removeUnknownStops(
   }
 
   /**
-   * Check stop time updates of trip update that results in a new trip (ADDED or MODIFIED) and find
+   * Check stop time updates of trip update that results in a new trip (NEW or REPLACEMENT) and find
    * all stops of that trip.
    *
    * @return stops when stop time updates are correct; null if there are errors
@@ -582,11 +576,12 @@ private List<StopLocation> checkNewStopTimeUpdatesAndFindStops(
     final List<StopLocation> stops = new ArrayList<>(stopTimeUpdates.size());
 
     for (int index = 0; index < stopTimeUpdates.size(); ++index) {
-      final StopTimeUpdate stopTimeUpdate = stopTimeUpdates.get(index);
+      final var addedStopTime = new AddedStopTime(stopTimeUpdates.get(index));
 
       // Check stop sequence
-      if (stopTimeUpdate.hasStopSequence()) {
-        final Integer stopSequence = stopTimeUpdate.getStopSequence();
+      final var optionalStopSequence = addedStopTime.stopSequence();
+      if (optionalStopSequence.isPresent()) {
+        final var stopSequence = optionalStopSequence.getAsInt();
 
         // Check non-negative
         if (stopSequence < 0) {
@@ -601,14 +596,16 @@ private List<StopLocation> checkNewStopTimeUpdatesAndFindStops(
         }
         previousStopSequence = stopSequence;
       } else {
-        // Allow missing stop sequences for ADDED and MODIFIED trips
+        // Allow missing stop sequences for NEW and REPLACEMENT trips
       }
 
       // Find stops
-      if (stopTimeUpdate.hasStopId()) {
+      final var optionalStopId = addedStopTime.stopId();
+      if (optionalStopId.isPresent()) {
+        final var stopId = optionalStopId.get();
         // Find stop
         final var stop = transitEditorService.getRegularStop(
-          new FeedScopedId(tripId.getFeedId(), stopTimeUpdate.getStopId())
+          new FeedScopedId(tripId.getFeedId(), stopId)
         );
         if (stop != null) {
           // Remember stop
@@ -618,7 +615,7 @@ private List<StopLocation> checkNewStopTimeUpdatesAndFindStops(
             tripId,
             serviceDate,
             "Graph doesn't contain stop id '{}' of trip update, skipping.",
-            stopTimeUpdate.getStopId()
+            stopId
           );
           return null;
         }
@@ -633,9 +630,10 @@ private List<StopLocation> checkNewStopTimeUpdatesAndFindStops(
       }
 
       // Check arrival time
-      if (stopTimeUpdate.hasArrival() && stopTimeUpdate.getArrival().hasTime()) {
+      final var arrival = addedStopTime.arrivalTime();
+      if (arrival.isPresent()) {
+        final var time = arrival.getAsLong();
         // Check for increasing time
-        final Long time = stopTimeUpdate.getArrival().getTime();
         if (previousTime != null && previousTime > time) {
           debug(tripId, serviceDate, "Trip update contains decreasing times, skipping.");
           return null;
@@ -647,9 +645,10 @@ private List<StopLocation> checkNewStopTimeUpdatesAndFindStops(
       }
 
       // Check departure time
-      if (stopTimeUpdate.hasDeparture() && stopTimeUpdate.getDeparture().hasTime()) {
+      final var departure = addedStopTime.departureTime();
+      if (departure.isPresent()) {
+        final var time = departure.getAsLong();
         // Check for increasing time
-        final Long time = stopTimeUpdate.getDeparture().getTime();
         if (previousTime != null && previousTime > time) {
           debug(tripId, serviceDate, "Trip update contains decreasing times, skipping.");
           return null;
@@ -664,7 +663,7 @@ private List<StopLocation> checkNewStopTimeUpdatesAndFindStops(
   }
 
   /**
-   * Handle GTFS-RT TripUpdate message containing an ADDED trip.
+   * Handle GTFS-RT TripUpdate message containing an NEW trip.
    *
    * @param stopTimeUpdates GTFS-RT stop time updates
    * @param tripDescriptor  GTFS-RT TripDescriptor
@@ -713,7 +712,7 @@ private Result<UpdateSuccess, UpdateError> handleAddedTrip(
       debug(
         tripId,
         serviceDate,
-        "ADDED trip has service date {} for which no service id is available, skipping.",
+        "NEW trip has service date {} for which no service id is available, skipping.",
         serviceDate.toString()
       );
       return UpdateError.result(tripId, NO_SERVICE_ON_DATE);
@@ -721,6 +720,12 @@ private Result<UpdateSuccess, UpdateError> handleAddedTrip(
       // Just use first service id of set
       tripBuilder.withServiceId(serviceIds.iterator().next());
     }
+
+    var tripHeadsign = getTripHeadsign(tripUpdate);
+    if (tripHeadsign != null) {
+      tripBuilder.withHeadsign(new NonLocalizedString(tripHeadsign));
+    }
+
     return addTripToGraphAndBuffer(
       tripBuilder.build(),
       tripUpdate.getVehicle(),
@@ -728,10 +733,22 @@ private Result<UpdateSuccess, UpdateError> handleAddedTrip(
       stops,
       serviceDate,
       RealTimeState.ADDED,
-      !routeExists
+      !routeExists,
+      tripHeadsign
     );
   }
 
+  @Nullable
+  private static String getTripHeadsign(TripUpdate tripUpdate) {
+    if (tripUpdate.hasTripProperties()) {
+      var tripProperties = tripUpdate.getTripProperties();
+      if (tripProperties.hasTripHeadsign()) {
+        return tripProperties.getTripHeadsign();
+      }
+    }
+    return null;
+  }
+
   private Route createRoute(TripDescriptor tripDescriptor, FeedScopedId tripId) {
     // the route in this update doesn't already exist, but the update contains the information so it will be created
     if (
@@ -813,7 +830,8 @@ private Result<UpdateSuccess, UpdateError> addTripToGraphAndBuffer(
     final List<StopLocation> stops,
     final LocalDate serviceDate,
     final RealTimeState realTimeState,
-    final boolean isAddedRoute
+    final boolean isAddedRoute,
+    @Nullable final String tripHeadsign
   ) {
     // Preconditions
     Objects.requireNonNull(stops);
@@ -830,7 +848,7 @@ private Result<UpdateSuccess, UpdateError> addTripToGraphAndBuffer(
     // Create StopTimes
     final List<StopTime> stopTimes = new ArrayList<>(stopTimeUpdates.size());
     for (int index = 0; index < stopTimeUpdates.size(); ++index) {
-      final StopTimeUpdate stopTimeUpdate = stopTimeUpdates.get(index);
+      final var added = new AddedStopTime(stopTimeUpdates.get(index));
       final var stop = stops.get(index);
 
       // Create stop time
@@ -838,13 +856,15 @@ private Result<UpdateSuccess, UpdateError> addTripToGraphAndBuffer(
       stopTime.setTrip(trip);
       stopTime.setStop(stop);
       // Set arrival time
-      if (stopTimeUpdate.hasArrival() && stopTimeUpdate.getArrival().hasTime()) {
-        final long arrivalTime = stopTimeUpdate.getArrival().getTime() - midnightSecondsSinceEpoch;
+      final var arrival = added.arrivalTime();
+      if (arrival.isPresent()) {
+        final var delay = added.arrivalDelay();
+        final var arrivalTime = arrival.getAsLong() - midnightSecondsSinceEpoch - delay;
         if (arrivalTime < 0 || arrivalTime > MAX_ARRIVAL_DEPARTURE_TIME) {
           debug(
             trip.getId(),
             serviceDate,
-            "ADDED trip has invalid arrival time (compared to start date in " +
+            "NEW trip has invalid arrival time (compared to start date in " +
             "TripDescriptor), skipping."
           );
           return UpdateError.result(trip.getId(), INVALID_ARRIVAL_TIME);
@@ -852,14 +872,15 @@ private Result<UpdateSuccess, UpdateError> addTripToGraphAndBuffer(
         stopTime.setArrivalTime((int) arrivalTime);
       }
       // Set departure time
-      if (stopTimeUpdate.hasDeparture() && stopTimeUpdate.getDeparture().hasTime()) {
-        final long departureTime =
-          stopTimeUpdate.getDeparture().getTime() - midnightSecondsSinceEpoch;
+      final var departure = added.departureTime();
+      if (departure.isPresent()) {
+        final var delay = added.departureDelay();
+        final long departureTime = departure.getAsLong() - midnightSecondsSinceEpoch - delay;
         if (departureTime < 0 || departureTime > MAX_ARRIVAL_DEPARTURE_TIME) {
           debug(
             trip.getId(),
             serviceDate,
-            "ADDED trip has invalid departure time (compared to start date in " +
+            "NEW trip has invalid departure time (compared to start date in " +
             "TripDescriptor), skipping."
           );
           return UpdateError.result(trip.getId(), INVALID_DEPARTURE_TIME);
@@ -867,12 +888,19 @@ private Result<UpdateSuccess, UpdateError> addTripToGraphAndBuffer(
         stopTime.setDepartureTime((int) departureTime);
       }
       stopTime.setTimepoint(1); // Exact time
-      if (stopTimeUpdate.hasStopSequence()) {
-        stopTime.setStopSequence(stopTimeUpdate.getStopSequence());
-      }
-      var added = AddedStopTime.ofStopTime(stopTimeUpdate);
+      added.stopSequence().ifPresent(stopTime::setStopSequence);
       stopTime.setPickupType(added.pickup());
       stopTime.setDropOffType(added.dropOff());
+      added
+        .stopHeadsign()
+        .ifPresentOrElse(
+          headsign -> stopTime.setStopHeadsign(new NonLocalizedString(headsign)),
+          () -> {
+            if (tripHeadsign != null) {
+              stopTime.setStopHeadsign(new NonLocalizedString(tripHeadsign));
+            }
+          }
+        );
       // Add stop time to list
       stopTimes.add(stopTime);
     }
@@ -898,12 +926,23 @@ private Result<UpdateSuccess, UpdateError> addTripToGraphAndBuffer(
     );
 
     // Update all times to mark trip times as realtime
-    // TODO: should we incorporate the delay field if present?
+    // TODO: This is based on the proposal at https://github.com/google/transit/issues/490
     for (int stopIndex = 0; stopIndex < newTripTimes.getNumStops(); stopIndex++) {
-      newTripTimes.updateArrivalTime(stopIndex, newTripTimes.getScheduledArrivalTime(stopIndex));
+      final var addedStopTime = new AddedStopTime(stopTimeUpdates.get(stopIndex));
+
+      if (addedStopTime.isSkipped()) {
+        newTripTimes.setCancelled(stopIndex);
+      }
+
+      final int arrivalDelay = addedStopTime.arrivalDelay();
+      final int departureDelay = addedStopTime.departureDelay();
+      newTripTimes.updateArrivalTime(
+        stopIndex,
+        newTripTimes.getScheduledArrivalTime(stopIndex) + arrivalDelay
+      );
       newTripTimes.updateDepartureTime(
         stopIndex,
-        newTripTimes.getScheduledDepartureTime(stopIndex)
+        newTripTimes.getScheduledDepartureTime(stopIndex) + departureDelay
       );
     }
 
@@ -1026,7 +1065,7 @@ private boolean cancelPreviouslyAddedTrip(
   }
 
   /**
-   * Validate and handle GTFS-RT TripUpdate message containing a MODIFIED trip.
+   * Validate and handle GTFS-RT TripUpdate message containing a REPLACEMENT trip.
    *
    * @param tripUpdate     GTFS-RT TripUpdate message
    * @param tripDescriptor GTFS-RT TripDescriptor
@@ -1050,8 +1089,7 @@ private Result<UpdateSuccess, UpdateError> validateAndHandleModifiedTrip(
     Trip trip = transitEditorService.getTrip(tripId);
 
     if (trip == null) {
-      // TODO: should we support this and consider it an ADDED trip?
-      debug(tripId, serviceDate, "Feed does not contain trip id of MODIFIED trip, skipping.");
+      debug(tripId, serviceDate, "Feed does not contain trip id of REPLACEMENT trip, skipping.");
       return UpdateError.result(tripId, TRIP_NOT_FOUND);
     }
 
@@ -1137,7 +1175,8 @@ private Result<UpdateSuccess, UpdateError> handleModifiedTrip(
       stops,
       serviceDate,
       RealTimeState.MODIFIED,
-      false
+      false,
+      getTripHeadsign(tripUpdate)
     );
   }
 
diff --git a/application/src/test/java/org/opentripplanner/updater/trip/RealtimeTestConstants.java b/application/src/test/java/org/opentripplanner/updater/trip/RealtimeTestConstants.java
index 87a9da83911..f34931a2a98 100644
--- a/application/src/test/java/org/opentripplanner/updater/trip/RealtimeTestConstants.java
+++ b/application/src/test/java/org/opentripplanner/updater/trip/RealtimeTestConstants.java
@@ -18,6 +18,7 @@ public interface RealtimeTestConstants {
   String STOP_A1_ID = "A1";
   String STOP_B1_ID = "B1";
   String STOP_C1_ID = "C1";
+  String STOP_D1_ID = "D1";
   String TRIP_1_ID = "TestTrip1";
   String TRIP_2_ID = "TestTrip2";
   String OPERATOR_1_ID = "TestOperator1";
diff --git a/application/src/test/java/org/opentripplanner/updater/trip/RealtimeTestEnvironmentBuilder.java b/application/src/test/java/org/opentripplanner/updater/trip/RealtimeTestEnvironmentBuilder.java
index eb31a555e1d..4f60d8de40d 100644
--- a/application/src/test/java/org/opentripplanner/updater/trip/RealtimeTestEnvironmentBuilder.java
+++ b/application/src/test/java/org/opentripplanner/updater/trip/RealtimeTestEnvironmentBuilder.java
@@ -58,11 +58,11 @@ public RealtimeTestEnvironment build() {
     return new RealtimeTestEnvironment(sourceType, timetableRepository);
   }
 
-  private Trip createTrip(TripInput tripInput) {
+  void createTrip(TripInput tripInput) {
     var trip = Trip
       .of(id(tripInput.id()))
       .withRoute(tripInput.route())
-      .withHeadsign(I18NString.of("Headsign of %s".formatted(tripInput.id())))
+      .withHeadsign(tripInput.headsign() == null ? null : I18NString.of(tripInput.headsign()))
       .withServiceId(SERVICE_ID)
       .build();
 
@@ -99,8 +99,6 @@ private Trip createTrip(TripInput tripInput) {
       .build();
 
     timetableRepository.addTripPattern(pattern.getId(), pattern);
-
-    return trip;
   }
 
   private static StopTime createStopTime(
diff --git a/application/src/test/java/org/opentripplanner/updater/trip/TimetableSnapshotSourceTest.java b/application/src/test/java/org/opentripplanner/updater/trip/TimetableSnapshotSourceTest.java
index d0e6f19a156..fe6a9abb7fa 100644
--- a/application/src/test/java/org/opentripplanner/updater/trip/TimetableSnapshotSourceTest.java
+++ b/application/src/test/java/org/opentripplanner/updater/trip/TimetableSnapshotSourceTest.java
@@ -1,21 +1,12 @@
 package org.opentripplanner.updater.trip;
 
 import static com.google.transit.realtime.GtfsRealtime.TripDescriptor.ScheduleRelationship.CANCELED;
-import static com.google.transit.realtime.GtfsRealtime.TripUpdate.StopTimeUpdate.ScheduleRelationship.SKIPPED;
-import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
-import static org.junit.jupiter.api.Assertions.assertNotSame;
 import static org.junit.jupiter.api.Assertions.assertSame;
-import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.opentripplanner.updater.trip.BackwardsDelayPropagationType.REQUIRED_NO_DATA;
 import static org.opentripplanner.updater.trip.UpdateIncrementality.DIFFERENTIAL;
 
-import com.google.transit.realtime.GtfsRealtime.TripDescriptor;
-import com.google.transit.realtime.GtfsRealtime.TripDescriptor.ScheduleRelationship;
 import com.google.transit.realtime.GtfsRealtime.TripUpdate;
-import com.google.transit.realtime.GtfsRealtime.TripUpdate.StopTimeEvent;
-import com.google.transit.realtime.GtfsRealtime.TripUpdate.StopTimeUpdate;
 import java.time.Duration;
 import java.time.LocalDate;
 import java.util.List;
@@ -24,19 +15,12 @@
 import org.opentripplanner.ConstantsForTests;
 import org.opentripplanner.TestOtpModel;
 import org.opentripplanner._support.time.ZoneIds;
-import org.opentripplanner.model.Timetable;
 import org.opentripplanner.model.TimetableSnapshot;
-import org.opentripplanner.transit.model.framework.FeedScopedId;
-import org.opentripplanner.transit.model.network.TripPattern;
-import org.opentripplanner.transit.model.timetable.RealTimeState;
-import org.opentripplanner.transit.model.timetable.Trip;
-import org.opentripplanner.transit.model.timetable.TripTimes;
 import org.opentripplanner.transit.service.DefaultTransitService;
 import org.opentripplanner.transit.service.TimetableRepository;
 import org.opentripplanner.transit.service.TransitService;
 import org.opentripplanner.updater.GtfsRealtimeFuzzyTripMatcher;
 import org.opentripplanner.updater.TimetableSnapshotSourceParameters;
-import org.opentripplanner.utils.time.ServiceDateUtils;
 
 public class TimetableSnapshotSourceTest {
 
@@ -81,206 +65,6 @@ public void testGetSnapshot() {
     assertSame(snapshot, updater.getTimetableSnapshot());
   }
 
-  @Test
-  public void testHandleModifiedTrip() {
-    // GIVEN
-
-    String modifiedTripId = "10.1";
-
-    TripUpdate tripUpdate;
-    {
-      final TripDescriptor.Builder tripDescriptorBuilder = TripDescriptor.newBuilder();
-
-      tripDescriptorBuilder.setTripId(modifiedTripId);
-      tripDescriptorBuilder.setScheduleRelationship(ScheduleRelationship.REPLACEMENT);
-      tripDescriptorBuilder.setStartDate(ServiceDateUtils.asCompactString(SERVICE_DATE));
-
-      final long midnightSecondsSinceEpoch = ServiceDateUtils
-        .asStartOfService(SERVICE_DATE, transitService.getTimeZone())
-        .toEpochSecond();
-
-      final TripUpdate.Builder tripUpdateBuilder = TripUpdate.newBuilder();
-
-      tripUpdateBuilder.setTrip(tripDescriptorBuilder);
-
-      { // Stop O
-        final StopTimeUpdate.Builder stopTimeUpdateBuilder = tripUpdateBuilder.addStopTimeUpdateBuilder();
-        stopTimeUpdateBuilder.setScheduleRelationship(
-          StopTimeUpdate.ScheduleRelationship.SCHEDULED
-        );
-        stopTimeUpdateBuilder.setStopId("O");
-        stopTimeUpdateBuilder.setStopSequence(10);
-
-        { // Arrival
-          final StopTimeEvent.Builder arrivalBuilder = stopTimeUpdateBuilder.getArrivalBuilder();
-          arrivalBuilder.setTime(midnightSecondsSinceEpoch + (12 * 3600) + (30 * 60));
-          arrivalBuilder.setDelay(0);
-        }
-
-        { // Departure
-          final StopTimeEvent.Builder departureBuilder = stopTimeUpdateBuilder.getDepartureBuilder();
-          departureBuilder.setTime(midnightSecondsSinceEpoch + (12 * 3600) + (30 * 60));
-          departureBuilder.setDelay(0);
-        }
-      }
-
-      { // Stop C
-        final StopTimeUpdate.Builder stopTimeUpdateBuilder = tripUpdateBuilder.addStopTimeUpdateBuilder();
-        stopTimeUpdateBuilder.setScheduleRelationship(
-          StopTimeUpdate.ScheduleRelationship.SCHEDULED
-        );
-        stopTimeUpdateBuilder.setStopId("C");
-        stopTimeUpdateBuilder.setStopSequence(30);
-
-        { // Arrival
-          final StopTimeEvent.Builder arrivalBuilder = stopTimeUpdateBuilder.getArrivalBuilder();
-          arrivalBuilder.setTime(midnightSecondsSinceEpoch + (12 * 3600) + (40 * 60));
-          arrivalBuilder.setDelay(0);
-        }
-
-        { // Departure
-          final StopTimeEvent.Builder departureBuilder = stopTimeUpdateBuilder.getDepartureBuilder();
-          departureBuilder.setTime(midnightSecondsSinceEpoch + (12 * 3600) + (45 * 60));
-          departureBuilder.setDelay(0);
-        }
-      }
-
-      { // Stop D
-        final StopTimeUpdate.Builder stopTimeUpdateBuilder = tripUpdateBuilder.addStopTimeUpdateBuilder();
-        stopTimeUpdateBuilder.setScheduleRelationship(SKIPPED);
-        stopTimeUpdateBuilder.setStopId("D");
-        stopTimeUpdateBuilder.setStopSequence(40);
-
-        { // Arrival
-          final StopTimeEvent.Builder arrivalBuilder = stopTimeUpdateBuilder.getArrivalBuilder();
-          arrivalBuilder.setTime(midnightSecondsSinceEpoch + (12 * 3600) + (50 * 60));
-          arrivalBuilder.setDelay(0);
-        }
-
-        { // Departure
-          final StopTimeEvent.Builder departureBuilder = stopTimeUpdateBuilder.getDepartureBuilder();
-          departureBuilder.setTime(midnightSecondsSinceEpoch + (12 * 3600) + (51 * 60));
-          departureBuilder.setDelay(0);
-        }
-      }
-
-      { // Stop P
-        final StopTimeUpdate.Builder stopTimeUpdateBuilder = tripUpdateBuilder.addStopTimeUpdateBuilder();
-        stopTimeUpdateBuilder.setScheduleRelationship(
-          StopTimeUpdate.ScheduleRelationship.SCHEDULED
-        );
-        stopTimeUpdateBuilder.setStopId("P");
-        stopTimeUpdateBuilder.setStopSequence(50);
-
-        { // Arrival
-          final StopTimeEvent.Builder arrivalBuilder = stopTimeUpdateBuilder.getArrivalBuilder();
-          arrivalBuilder.setTime(midnightSecondsSinceEpoch + (12 * 3600) + (55 * 60));
-          arrivalBuilder.setDelay(0);
-        }
-
-        { // Departure
-          final StopTimeEvent.Builder departureBuilder = stopTimeUpdateBuilder.getDepartureBuilder();
-          departureBuilder.setTime(midnightSecondsSinceEpoch + (12 * 3600) + (55 * 60));
-          departureBuilder.setDelay(0);
-        }
-      }
-
-      tripUpdate = tripUpdateBuilder.build();
-    }
-
-    var updater = defaultUpdater();
-
-    // WHEN
-    updater.applyTripUpdates(
-      TRIP_MATCHER_NOOP,
-      REQUIRED_NO_DATA,
-      DIFFERENTIAL,
-      List.of(tripUpdate),
-      feedId
-    );
-    updater.flushBuffer();
-
-    // THEN
-    final TimetableSnapshot snapshot = updater.getTimetableSnapshot();
-
-    // Original trip pattern
-    {
-      final FeedScopedId tripId = new FeedScopedId(feedId, modifiedTripId);
-      final Trip trip = transitService.getTrip(tripId);
-      final TripPattern originalTripPattern = transitService.findPattern(trip);
-
-      final Timetable originalTimetableForToday = snapshot.resolve(
-        originalTripPattern,
-        SERVICE_DATE
-      );
-      final Timetable originalTimetableScheduled = snapshot.resolve(originalTripPattern, null);
-
-      assertNotSame(originalTimetableForToday, originalTimetableScheduled);
-
-      final int originalTripIndexScheduled = originalTimetableScheduled.getTripIndex(
-        modifiedTripId
-      );
-      assertTrue(
-        originalTripIndexScheduled > -1,
-        "Original trip should be found in scheduled time table"
-      );
-      final TripTimes originalTripTimesScheduled = originalTimetableScheduled.getTripTimes(
-        originalTripIndexScheduled
-      );
-      assertFalse(
-        originalTripTimesScheduled.isCanceledOrDeleted(),
-        "Original trip times should not be canceled in scheduled time table"
-      );
-      assertEquals(RealTimeState.SCHEDULED, originalTripTimesScheduled.getRealTimeState());
-
-      final int originalTripIndexForToday = originalTimetableForToday.getTripIndex(modifiedTripId);
-      assertTrue(
-        originalTripIndexForToday > -1,
-        "Original trip should be found in time table for service date"
-      );
-      final TripTimes originalTripTimesForToday = originalTimetableForToday.getTripTimes(
-        originalTripIndexForToday
-      );
-      assertTrue(
-        originalTripTimesForToday.isDeleted(),
-        "Original trip times should be deleted in time table for service date"
-      );
-      assertEquals(RealTimeState.DELETED, originalTripTimesForToday.getRealTimeState());
-    }
-
-    // New trip pattern
-    {
-      final TripPattern newTripPattern = snapshot.getNewTripPatternForModifiedTrip(
-        new FeedScopedId(feedId, modifiedTripId),
-        SERVICE_DATE
-      );
-      assertNotNull(newTripPattern, "New trip pattern should be found");
-
-      final Timetable newTimetableForToday = snapshot.resolve(newTripPattern, SERVICE_DATE);
-      final Timetable newTimetableScheduled = snapshot.resolve(newTripPattern, null);
-
-      assertNotSame(newTimetableForToday, newTimetableScheduled);
-
-      final int newTimetableForTodayModifiedTripIndex = newTimetableForToday.getTripIndex(
-        modifiedTripId
-      );
-      assertTrue(
-        newTimetableForTodayModifiedTripIndex > -1,
-        "New trip should be found in time table for service date"
-      );
-      assertEquals(
-        RealTimeState.MODIFIED,
-        newTimetableForToday.getTripTimes(newTimetableForTodayModifiedTripIndex).getRealTimeState()
-      );
-
-      assertEquals(
-        -1,
-        newTimetableScheduled.getTripIndex(modifiedTripId),
-        "New trip should not be found in scheduled time table"
-      );
-    }
-  }
-
   private TimetableSnapshotSource defaultUpdater() {
     return new TimetableSnapshotSource(
       new TimetableSnapshotSourceParameters(Duration.ZERO, true),
diff --git a/application/src/test/java/org/opentripplanner/updater/trip/TripInput.java b/application/src/test/java/org/opentripplanner/updater/trip/TripInput.java
index eb4f3685659..18b45bc6c1d 100644
--- a/application/src/test/java/org/opentripplanner/updater/trip/TripInput.java
+++ b/application/src/test/java/org/opentripplanner/updater/trip/TripInput.java
@@ -2,6 +2,7 @@
 
 import java.util.ArrayList;
 import java.util.List;
+import javax.annotation.Nullable;
 import org.opentripplanner.transit.model.network.Route;
 import org.opentripplanner.transit.model.site.RegularStop;
 import org.opentripplanner.utils.time.TimeUtils;
@@ -10,7 +11,7 @@
  * A simple data structure that is used by the {@link RealtimeTestEnvironment} to create
  * trips, trips on date and patterns.
  */
-public record TripInput(String id, Route route, List<StopCall> stops) {
+public record TripInput(String id, Route route, List<StopCall> stops, @Nullable String headsign) {
   public static TripInputBuilder of(String id) {
     return new TripInputBuilder(id);
   }
@@ -22,6 +23,9 @@ public static class TripInputBuilder implements RealtimeTestConstants {
     // can be made configurable if needed
     private Route route = ROUTE_1;
 
+    @Nullable
+    private String headsign;
+
     TripInputBuilder(String id) {
       this.id = id;
     }
@@ -34,13 +38,18 @@ public TripInputBuilder addStop(RegularStop stopId, String arrivalTime, String d
     }
 
     public TripInput build() {
-      return new TripInput(id, route, stops);
+      return new TripInput(id, route, stops, headsign);
     }
 
     public TripInputBuilder withRoute(Route route) {
       this.route = route;
       return this;
     }
+
+    public TripInputBuilder withHeadsign(String headsign) {
+      this.headsign = headsign;
+      return this;
+    }
   }
 
   record StopCall(RegularStop stop, int arrivalTime, int departureTime) {}
diff --git a/application/src/test/java/org/opentripplanner/updater/trip/TripUpdateBuilder.java b/application/src/test/java/org/opentripplanner/updater/trip/TripUpdateBuilder.java
index e8218edfc1f..3606d9f22ef 100644
--- a/application/src/test/java/org/opentripplanner/updater/trip/TripUpdateBuilder.java
+++ b/application/src/test/java/org/opentripplanner/updater/trip/TripUpdateBuilder.java
@@ -7,6 +7,7 @@
 import java.time.LocalDate;
 import java.time.ZoneId;
 import java.time.ZonedDateTime;
+import javax.annotation.Nullable;
 import org.opentripplanner.utils.time.ServiceDateUtils;
 
 public class TripUpdateBuilder {
@@ -37,32 +38,134 @@ public TripUpdateBuilder(
     this.midnight = ServiceDateUtils.asStartOfService(serviceDate, zoneId);
   }
 
-  public TripUpdateBuilder addStopTime(String stopId, int minutes) {
+  public TripUpdateBuilder(
+    String tripId,
+    LocalDate serviceDate,
+    GtfsRealtime.TripDescriptor.ScheduleRelationship scheduleRelationship,
+    ZoneId zoneId,
+    String tripHeadsign
+  ) {
+    this(tripId, serviceDate, scheduleRelationship, zoneId);
+    tripUpdateBuilder.setTripProperties(
+      GtfsRealtime.TripUpdate.TripProperties.newBuilder().setTripHeadsign(tripHeadsign).build()
+    );
+  }
+
+  public TripUpdateBuilder addStopTime(String stopId, int secondsFromMidnight) {
+    return addStopTime(
+      stopId,
+      secondsFromMidnight,
+      NO_VALUE,
+      NO_DELAY,
+      NO_DELAY,
+      DEFAULT_SCHEDULE_RELATIONSHIP,
+      null,
+      null,
+      null,
+      NO_VALUE
+    );
+  }
+
+  public TripUpdateBuilder addStopTime(String stopId, int secondsFromMidnight, String headsign) {
+    return addStopTime(
+      stopId,
+      secondsFromMidnight,
+      NO_VALUE,
+      NO_DELAY,
+      NO_DELAY,
+      DEFAULT_SCHEDULE_RELATIONSHIP,
+      null,
+      null,
+      headsign,
+      NO_VALUE
+    );
+  }
+
+  public TripUpdateBuilder addStopTimeWithDelay(String stopId, int secondsFromMidnight, int delay) {
+    return addStopTime(
+      stopId,
+      secondsFromMidnight,
+      NO_VALUE,
+      delay,
+      delay,
+      DEFAULT_SCHEDULE_RELATIONSHIP,
+      null,
+      null,
+      null,
+      NO_VALUE
+    );
+  }
+
+  public TripUpdateBuilder addStopTimeWithScheduled(
+    String stopId,
+    int secondsFromMidnight,
+    int scheduledSeconds
+  ) {
+    return addStopTime(
+      stopId,
+      secondsFromMidnight,
+      NO_VALUE,
+      NO_DELAY,
+      NO_DELAY,
+      DEFAULT_SCHEDULE_RELATIONSHIP,
+      null,
+      null,
+      null,
+      scheduledSeconds
+    );
+  }
+
+  public TripUpdateBuilder addStopTime(
+    String stopId,
+    int secondsFromMidnight,
+    DropOffPickupType pickDrop
+  ) {
     return addStopTime(
       stopId,
-      minutes,
+      secondsFromMidnight,
       NO_VALUE,
       NO_DELAY,
       NO_DELAY,
       DEFAULT_SCHEDULE_RELATIONSHIP,
-      null
+      pickDrop,
+      null,
+      null,
+      NO_VALUE
     );
   }
 
-  public TripUpdateBuilder addStopTime(String stopId, int minutes, DropOffPickupType pickDrop) {
+  public TripUpdateBuilder addStopTime(
+    String stopId,
+    int secondsFromMidnight,
+    StopTimeUpdate.StopTimeProperties.DropOffPickupType pickDrop
+  ) {
     return addStopTime(
       stopId,
-      minutes,
+      secondsFromMidnight,
       NO_VALUE,
       NO_DELAY,
       NO_DELAY,
       DEFAULT_SCHEDULE_RELATIONSHIP,
-      pickDrop
+      null,
+      pickDrop,
+      null,
+      NO_VALUE
     );
   }
 
   public TripUpdateBuilder addDelayedStopTime(int stopSequence, int delay) {
-    return addStopTime(null, -1, stopSequence, delay, delay, DEFAULT_SCHEDULE_RELATIONSHIP, null);
+    return addStopTime(
+      null,
+      -1,
+      stopSequence,
+      delay,
+      delay,
+      DEFAULT_SCHEDULE_RELATIONSHIP,
+      null,
+      null,
+      null,
+      NO_VALUE
+    );
   }
 
   public TripUpdateBuilder addDelayedStopTime(
@@ -77,7 +180,10 @@ public TripUpdateBuilder addDelayedStopTime(
       arrivalDelay,
       departureDelay,
       DEFAULT_SCHEDULE_RELATIONSHIP,
-      null
+      null,
+      null,
+      null,
+      NO_VALUE
     );
   }
 
@@ -92,7 +198,10 @@ public TripUpdateBuilder addNoDataStop(int stopSequence) {
       NO_DELAY,
       NO_DELAY,
       StopTimeUpdate.ScheduleRelationship.NO_DATA,
-      null
+      null,
+      null,
+      null,
+      NO_VALUE
     );
   }
 
@@ -107,7 +216,44 @@ public TripUpdateBuilder addSkippedStop(int stopSequence) {
       NO_DELAY,
       NO_DELAY,
       StopTimeUpdate.ScheduleRelationship.SKIPPED,
-      null
+      null,
+      null,
+      null,
+      NO_VALUE
+    );
+  }
+
+  public TripUpdateBuilder addSkippedStop(String stopId, int secondsFromMidnight) {
+    return addStopTime(
+      stopId,
+      secondsFromMidnight,
+      NO_VALUE,
+      NO_DELAY,
+      NO_DELAY,
+      StopTimeUpdate.ScheduleRelationship.SKIPPED,
+      null,
+      null,
+      null,
+      NO_VALUE
+    );
+  }
+
+  public TripUpdateBuilder addSkippedStop(
+    String stopId,
+    int secondsFromMidnight,
+    DropOffPickupType pickDrop
+  ) {
+    return addStopTime(
+      stopId,
+      secondsFromMidnight,
+      NO_VALUE,
+      NO_DELAY,
+      NO_DELAY,
+      StopTimeUpdate.ScheduleRelationship.SKIPPED,
+      pickDrop,
+      null,
+      null,
+      NO_VALUE
     );
   }
 
@@ -121,13 +267,16 @@ public TripUpdateBuilder addRawStopTime(StopTimeUpdate stopTime) {
   }
 
   private TripUpdateBuilder addStopTime(
-    String stopId,
-    int minutes,
+    @Nullable String stopId,
+    int secondsFromMidnight,
     int stopSequence,
     int arrivalDelay,
     int departureDelay,
     StopTimeUpdate.ScheduleRelationship scheduleRelationShip,
-    DropOffPickupType pickDrop
+    @Nullable DropOffPickupType pickDrop,
+    @Nullable StopTimeUpdate.StopTimeProperties.DropOffPickupType gtfsPickDrop,
+    @Nullable String headsign,
+    int scheduledSeconds
   ) {
     final StopTimeUpdate.Builder stopTimeUpdateBuilder = tripUpdateBuilder.addStopTimeUpdateBuilder();
     stopTimeUpdateBuilder.setScheduleRelationship(scheduleRelationShip);
@@ -140,25 +289,41 @@ private TripUpdateBuilder addStopTime(
       stopTimeUpdateBuilder.setStopSequence(stopSequence);
     }
 
-    if (pickDrop != null) {
+    if (pickDrop != null || gtfsPickDrop != null || headsign != null) {
       var stopTimePropsBuilder = stopTimeUpdateBuilder.getStopTimePropertiesBuilder();
-      var b = MfdzRealtimeExtensions.StopTimePropertiesExtension.newBuilder();
-      b.setDropoffType(pickDrop);
-      b.setPickupType(pickDrop);
 
-      var ext = b.build();
-      stopTimePropsBuilder.setExtension(MfdzRealtimeExtensions.stopTimeProperties, ext);
+      if (headsign != null) {
+        stopTimePropsBuilder.setStopHeadsign(headsign);
+      }
+
+      if (gtfsPickDrop != null) {
+        stopTimePropsBuilder.setDropOffType(gtfsPickDrop);
+        stopTimePropsBuilder.setPickupType(gtfsPickDrop);
+      }
+      if (pickDrop != null) {
+        var b = MfdzRealtimeExtensions.StopTimePropertiesExtension.newBuilder();
+        b.setDropoffType(pickDrop);
+        b.setPickupType(pickDrop);
+        var ext = b.build();
+        stopTimePropsBuilder.setExtension(MfdzRealtimeExtensions.stopTimeProperties, ext);
+      }
     }
 
     final GtfsRealtime.TripUpdate.StopTimeEvent.Builder arrivalBuilder = stopTimeUpdateBuilder.getArrivalBuilder();
     final GtfsRealtime.TripUpdate.StopTimeEvent.Builder departureBuilder = stopTimeUpdateBuilder.getDepartureBuilder();
 
-    if (minutes > NO_VALUE) {
-      var epochSeconds = midnight.plusHours(8).plusMinutes(minutes).toEpochSecond();
+    if (secondsFromMidnight > NO_VALUE) {
+      var epochSeconds = midnight.plusSeconds(secondsFromMidnight).toEpochSecond();
       arrivalBuilder.setTime(epochSeconds);
       departureBuilder.setTime(epochSeconds);
     }
 
+    if (scheduledSeconds > NO_VALUE) {
+      var epochSeconds = midnight.plusSeconds(scheduledSeconds).toEpochSecond();
+      arrivalBuilder.setScheduledTime(epochSeconds);
+      departureBuilder.setScheduledTime(epochSeconds);
+    }
+
     if (arrivalDelay != NO_DELAY) {
       arrivalBuilder.setDelay(arrivalDelay);
     }
diff --git a/application/src/test/java/org/opentripplanner/updater/trip/moduletests/addition/AddedTest.java b/application/src/test/java/org/opentripplanner/updater/trip/moduletests/addition/AddedTest.java
index e926361f4ea..2f3c11befc1 100644
--- a/application/src/test/java/org/opentripplanner/updater/trip/moduletests/addition/AddedTest.java
+++ b/application/src/test/java/org/opentripplanner/updater/trip/moduletests/addition/AddedTest.java
@@ -2,15 +2,19 @@
 
 import static com.google.transit.realtime.GtfsRealtime.TripDescriptor.ScheduleRelationship.ADDED;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNotSame;
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.opentripplanner.updater.spi.UpdateResultAssertions.assertSuccess;
 
+import com.google.transit.realtime.GtfsRealtime;
 import de.mfdz.MfdzRealtimeExtensions.StopTimePropertiesExtension.DropOffPickupType;
 import java.util.List;
+import java.util.Objects;
 import org.junit.jupiter.api.Test;
+import org.opentripplanner.framework.i18n.I18NString;
 import org.opentripplanner.model.PickDrop;
 import org.opentripplanner.transit.model._data.TimetableRepositoryForTest;
 import org.opentripplanner.transit.model.basic.TransitMode;
@@ -47,7 +51,11 @@ void addedTripWithNewRoute() {
     var tripUpdate = new TripUpdateBuilder(ADDED_TRIP_ID, SERVICE_DATE, ADDED, TIME_ZONE)
       .addTripExtension()
       .addStopTime(STOP_A1_ID, 30, DropOffPickupType.PHONE_AGENCY)
-      .addStopTime(STOP_B1_ID, 40, DropOffPickupType.COORDINATE_WITH_DRIVER)
+      .addStopTime(
+        STOP_B1_ID,
+        40,
+        GtfsRealtime.TripUpdate.StopTimeUpdate.StopTimeProperties.DropOffPickupType.COORDINATE_WITH_DRIVER
+      )
       .addStopTime(STOP_B1_ID, 55, DropOffPickupType.NONE)
       .build();
 
@@ -124,11 +132,82 @@ void repeatedlyAddedTripWithNewRoute() {
     assertNotNull(env.getTransitService().getRoute(firstRoute.getId()));
   }
 
-  private TripPattern assertAddedTrip(String tripId, RealtimeTestEnvironment env) {
+  @Test
+  public void addedTripWithSkippedStop() {
+    var env = RealtimeTestEnvironment.gtfs().build();
+    var builder = new TripUpdateBuilder(ADDED_TRIP_ID, SERVICE_DATE, ADDED, TIME_ZONE, "A loop");
+    builder
+      .addStopTime(STOP_A1_ID, 30, DropOffPickupType.PHONE_AGENCY)
+      .addSkippedStop(STOP_B1_ID, 40, DropOffPickupType.COORDINATE_WITH_DRIVER)
+      .addSkippedStop(STOP_C1_ID, 48)
+      .addStopTime(STOP_D1_ID, 55, "A (non-stop)")
+      .addStopTime(STOP_A1_ID, 60);
+    var tripUpdate = builder.build();
+
+    env.applyTripUpdate(tripUpdate);
+
+    // THEN
+    final TripPattern tripPattern = assertAddedTrip(ADDED_TRIP_ID, env);
+    assertEquals(PickDrop.CALL_AGENCY, tripPattern.getBoardType(0));
+    assertEquals(PickDrop.CANCELLED, tripPattern.getAlightType(1));
+    assertEquals(PickDrop.CANCELLED, tripPattern.getBoardType(1));
+    assertEquals(PickDrop.CANCELLED, tripPattern.getAlightType(2));
+    assertEquals(PickDrop.CANCELLED, tripPattern.getBoardType(2));
+    assertEquals(PickDrop.SCHEDULED, tripPattern.getAlightType(3));
+    var snapshot = env.getTimetableSnapshot();
+    var forToday = snapshot.resolve(tripPattern, SERVICE_DATE);
+    var forTodayAddedTripIndex = forToday.getTripIndex(ADDED_TRIP_ID);
+    var tripTimes = forToday.getTripTimes(forTodayAddedTripIndex);
+    var trip = env.getTransitService().getTrip(TimetableRepositoryForTest.id(ADDED_TRIP_ID));
+    assertEquals(I18NString.of("A loop"), Objects.requireNonNull(trip).getHeadsign());
+    assertEquals(I18NString.of("A loop"), tripTimes.getHeadsign(0));
+    assertFalse(tripTimes.isCancelledStop(0));
+    assertTrue(tripTimes.isCancelledStop(1));
+    assertTrue(tripTimes.isCancelledStop(2));
+    assertFalse(tripTimes.isCancelledStop(3));
+    assertEquals(I18NString.of("A (non-stop)"), tripTimes.getHeadsign(3));
+  }
+
+  @Test
+  public void addedTripWithDelay() {
+    var env = RealtimeTestEnvironment.gtfs().build();
+    var builder = new TripUpdateBuilder(ADDED_TRIP_ID, SERVICE_DATE, ADDED, TIME_ZONE);
+
+    builder
+      .addStopTime(STOP_A1_ID, 10000)
+      .addStopTimeWithDelay(STOP_B1_ID, 11300, 300)
+      .addStopTimeWithScheduled(STOP_C1_ID, 12500, 12000);
+
+    var tripUpdate = builder.build();
+    env.applyTripUpdate(tripUpdate);
+
+    // THEN
+    var tripPattern = assertAddedTrip(ADDED_TRIP_ID, env);
+    var snapshot = env.getTimetableSnapshot();
+    var forToday = snapshot.resolve(tripPattern, SERVICE_DATE);
+    var forTodayAddedTripIndex = forToday.getTripIndex(ADDED_TRIP_ID);
+    var tripTimes = forToday.getTripTimes(forTodayAddedTripIndex);
+    assertEquals(0, tripTimes.getDepartureDelay(0));
+    assertEquals(10000, tripTimes.getDepartureTime(0));
+    assertEquals(300, tripTimes.getArrivalDelay(1));
+    assertEquals(11300, tripTimes.getArrivalTime(1));
+    assertEquals(500, tripTimes.getArrivalDelay(2));
+    assertEquals(12500, tripTimes.getArrivalTime(2));
+  }
+
+  private static TripPattern assertAddedTrip(String tripId, RealtimeTestEnvironment env) {
+    return assertAddedTrip(tripId, env, RealTimeState.ADDED);
+  }
+
+  static TripPattern assertAddedTrip(
+    String tripId,
+    RealtimeTestEnvironment env,
+    RealTimeState realTimeState
+  ) {
     var snapshot = env.getTimetableSnapshot();
 
     TransitService transitService = env.getTransitService();
-    Trip trip = transitService.getTrip(TimetableRepositoryForTest.id(ADDED_TRIP_ID));
+    Trip trip = transitService.getTrip(TimetableRepositoryForTest.id(tripId));
     assertNotNull(trip);
     assertNotNull(transitService.findPattern(trip));
 
@@ -150,10 +229,7 @@ private TripPattern assertAddedTrip(String tripId, RealtimeTestEnvironment env)
       forTodayAddedTripIndex > -1,
       "Added trip should be found in time table for service date"
     );
-    assertEquals(
-      RealTimeState.ADDED,
-      forToday.getTripTimes(forTodayAddedTripIndex).getRealTimeState()
-    );
+    assertEquals(realTimeState, forToday.getTripTimes(forTodayAddedTripIndex).getRealTimeState());
 
     final int scheduleTripIndex = schedule.getTripIndex(tripId);
     assertEquals(-1, scheduleTripIndex, "Added trip should not be found in scheduled time table");
diff --git a/application/src/test/java/org/opentripplanner/updater/trip/moduletests/addition/ModifiedTest.java b/application/src/test/java/org/opentripplanner/updater/trip/moduletests/addition/ModifiedTest.java
new file mode 100644
index 00000000000..3ca93dbcae9
--- /dev/null
+++ b/application/src/test/java/org/opentripplanner/updater/trip/moduletests/addition/ModifiedTest.java
@@ -0,0 +1,132 @@
+package org.opentripplanner.updater.trip.moduletests.addition;
+
+import static com.google.transit.realtime.GtfsRealtime.TripDescriptor.ScheduleRelationship.REPLACEMENT;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNotSame;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.opentripplanner.updater.trip.moduletests.addition.AddedTest.assertAddedTrip;
+
+import org.junit.jupiter.api.Test;
+import org.opentripplanner.framework.i18n.I18NString;
+import org.opentripplanner.transit.model._data.TimetableRepositoryForTest;
+import org.opentripplanner.transit.model.timetable.RealTimeState;
+import org.opentripplanner.updater.trip.RealtimeTestConstants;
+import org.opentripplanner.updater.trip.RealtimeTestEnvironment;
+import org.opentripplanner.updater.trip.TripInput;
+import org.opentripplanner.updater.trip.TripUpdateBuilder;
+
+public class ModifiedTest implements RealtimeTestConstants {
+
+  @Test
+  void modifiedTrip() {
+    var TRIP_INPUT = TripInput
+      .of(TRIP_1_ID)
+      .addStop(STOP_A1, "8:30:00", "8:30:00")
+      .addStop(STOP_B1, "8:40:00", "8:40:00")
+      .withHeadsign("Original Headsign")
+      .build();
+    var env = RealtimeTestEnvironment.gtfs().addTrip(TRIP_INPUT).build();
+    var builder = new TripUpdateBuilder(
+      TRIP_1_ID,
+      SERVICE_DATE,
+      REPLACEMENT,
+      TIME_ZONE,
+      "New Headsign"
+    );
+    builder
+      .addStopTime(STOP_A1_ID, 30)
+      .addStopTime(STOP_B1_ID, 45, "Changed Headsign")
+      .addStopTime(STOP_C1_ID, 60);
+
+    var tripUpdate = builder.build();
+
+    env.applyTripUpdate(tripUpdate);
+
+    // THEN
+    var snapshot = env.getTimetableSnapshot();
+    var tripId = TimetableRepositoryForTest.id(TRIP_1_ID);
+
+    var transitService = env.getTransitService();
+    // We do not support trip headsign by service date
+    // TODO: I currently have no idea how TripOnServiceDate will behave, and will need to revisit this after #5393 is merged
+    assertEquals(
+      I18NString.of("Original Headsign"),
+      transitService.getTrip(TimetableRepositoryForTest.id(TRIP_1_ID)).getHeadsign()
+    );
+
+    // Original trip pattern
+    {
+      var trip = transitService.getTrip(tripId);
+      var originalTripPattern = transitService.findPattern(trip);
+
+      var originalTimetableForToday = snapshot.resolve(originalTripPattern, SERVICE_DATE);
+      var originalTimetableScheduled = snapshot.resolve(originalTripPattern, null);
+
+      assertNotSame(originalTimetableForToday, originalTimetableScheduled);
+
+      var originalTripIndexScheduled = originalTimetableScheduled.getTripIndex(TRIP_1_ID);
+      assertTrue(
+        originalTripIndexScheduled > -1,
+        "Original trip should be found in scheduled time table"
+      );
+      var originalTripTimesScheduled = originalTimetableScheduled.getTripTimes(
+        originalTripIndexScheduled
+      );
+      assertFalse(
+        originalTripTimesScheduled.isCanceledOrDeleted(),
+        "Original trip times should not be canceled in scheduled time table"
+      );
+      assertEquals(RealTimeState.SCHEDULED, originalTripTimesScheduled.getRealTimeState());
+
+      var originalTripIndexForToday = originalTimetableForToday.getTripIndex(TRIP_1_ID);
+      assertTrue(
+        originalTripIndexForToday > -1,
+        "Original trip should be found in time table for service date"
+      );
+      var originalTripTimesForToday = originalTimetableForToday.getTripTimes(
+        originalTripIndexForToday
+      );
+      assertTrue(
+        originalTripTimesForToday.isDeleted(),
+        "Original trip times should be deleted in time table for service date"
+      );
+      assertEquals(RealTimeState.DELETED, originalTripTimesForToday.getRealTimeState());
+      assertEquals(I18NString.of("Original Headsign"), originalTripTimesScheduled.getHeadsign(0));
+      assertEquals(I18NString.of("Original Headsign"), originalTripTimesScheduled.getHeadsign(1));
+      assertEquals(I18NString.of("Original Headsign"), originalTripTimesForToday.getHeadsign(0));
+      assertEquals(I18NString.of("Original Headsign"), originalTripTimesForToday.getHeadsign(1));
+    }
+
+    // New trip pattern
+    {
+      assertAddedTrip(TRIP_1_ID, env, RealTimeState.MODIFIED);
+      var newTripPattern = snapshot.getNewTripPatternForModifiedTrip(tripId, SERVICE_DATE);
+      assertNotNull(newTripPattern, "New trip pattern should be found");
+
+      var newTimetableForToday = snapshot.resolve(newTripPattern, SERVICE_DATE);
+      var newTimetableScheduled = snapshot.resolve(newTripPattern, null);
+
+      assertNotSame(newTimetableForToday, newTimetableScheduled);
+
+      var newTimetableForTodayModifiedTripIndex = newTimetableForToday.getTripIndex(TRIP_1_ID);
+      assertTrue(
+        newTimetableForTodayModifiedTripIndex > -1,
+        "New trip should be found in time table for service date"
+      );
+      var tripTimes = newTimetableForToday.getTripTimes(newTimetableForTodayModifiedTripIndex);
+      assertEquals(RealTimeState.MODIFIED, tripTimes.getRealTimeState());
+
+      assertEquals(
+        -1,
+        newTimetableScheduled.getTripIndex(TRIP_1_ID),
+        "New trip should not be found in scheduled time table"
+      );
+
+      assertEquals(I18NString.of("New Headsign"), tripTimes.getHeadsign(0));
+      assertEquals(I18NString.of("Changed Headsign"), tripTimes.getHeadsign(1));
+      assertEquals(I18NString.of("New Headsign"), tripTimes.getHeadsign(2));
+    }
+  }
+}
diff --git a/gtfs-realtime-protobuf/src/main/proto/gtfs-realtime.proto b/gtfs-realtime-protobuf/src/main/proto/gtfs-realtime.proto
index 6c3e11ee53f..bd610ae4243 100644
--- a/gtfs-realtime-protobuf/src/main/proto/gtfs-realtime.proto
+++ b/gtfs-realtime-protobuf/src/main/proto/gtfs-realtime.proto
@@ -106,6 +106,8 @@ message FeedEntity {
 
   // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
   optional Shape shape = 6;
+  optional Stop stop = 7;
+  optional TripModifications trip_modifications = 8;
 
   // The extensions namespace allows 3rd-party developers to extend the
   // GTFS Realtime Specification in order to add and evaluate new features and
@@ -188,6 +190,11 @@ message TripUpdate {
     // To specify a completely certain prediction, set its uncertainty to 0.
     optional int32 uncertainty = 3;
 
+    // Scheduled time for a new or replacement trip.
+    // In Unix time (i.e., number of seconds since January 1st 1970 00:00:00
+    // UTC).
+    optional int64 scheduled_time = 4;
+
     // The extensions namespace allows 3rd-party developers to extend the
     // GTFS Realtime Specification in order to add and evaluate new features
     // and modifications to the spec.
@@ -216,7 +223,7 @@ message TripUpdate {
     // Expected occupancy after departure from the given stop.
     // Should be provided only for future stops.
     // In order to provide departure_occupancy_status without either arrival or
-    // departure StopTimeEvents, ScheduleRelationship should be set to NO_DATA.
+    // departure StopTimeEvents, ScheduleRelationship should be set to NO_DATA. 
     optional VehiclePosition.OccupancyStatus departure_occupancy_status = 7;
 
     // The relation between the StopTimeEvents and the static schedule.
@@ -270,6 +277,32 @@ message TripUpdate {
       // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
       optional string assigned_stop_id = 1;
 
+      // The updated headsign of the vehicle at the stop.
+      // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+      optional string stop_headsign = 2;
+
+      enum DropOffPickupType {
+        // Regularly scheduled pickup/dropoff.
+        REGULAR = 0;
+
+        // No pickup/dropoff available
+        NONE = 1;
+
+        // Must phone agency to arrange pickup/dropoff.
+        PHONE_AGENCY = 2;
+
+        // Must coordinate with driver to arrange pickup/dropoff.
+        COORDINATE_WITH_DRIVER = 3;
+      }
+
+      // The updated pickup of the vehicle at the stop.
+      // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+      optional DropOffPickupType pickup_type = 3;
+
+      // The updated drop off of the vehicle at the stop.
+      // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+      optional DropOffPickupType drop_off_type = 4;
+
       // The extensions namespace allows 3rd-party developers to extend the
       // GTFS Realtime Specification in order to add and evaluate new features
       // and modifications to the spec.
@@ -340,7 +373,7 @@ message TripUpdate {
   optional int32 delay = 5;
 
   // Defines updated properties of the trip, such as a new shape_id when there is a detour. Or defines the
-  // trip_id, start_date, and start_time of a DUPLICATED trip.
+  // trip_id, start_date, and start_time of a DUPLICATED trip. 
   // NOTE: This message is still experimental, and subject to change. It may be formally adopted in the future.
   message TripProperties {
     // Defines the identifier of a new trip that is a duplicate of an existing trip defined in (CSV) GTFS trips.txt
@@ -374,9 +407,17 @@ message TripUpdate {
     // or a Shape in the (protobuf) real-time feed. The order of stops (stop sequences) for this trip must remain the same as
     // (CSV) GTFS. Stops that are a part of the original trip but will no longer be made, such as when a detour occurs, should
     // be marked as schedule_relationship=SKIPPED.
-    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future. 
     optional string shape_id = 4;
 
+    // Specifies the headsign for this trip when it differs from the original.
+    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+    optional string trip_headsign = 5;
+
+    // Specifies the name for this trip when it differs from the original.
+    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+    optional string trip_short_name = 6;
+
     // The extensions namespace allows 3rd-party developers to extend the
     // GTFS Realtime Specification in order to add and evaluate new features
     // and modifications to the spec.
@@ -450,7 +491,7 @@ message VehiclePosition {
   // The state of passenger occupancy for the vehicle or carriage.
   // Individual producers may not publish all OccupancyStatus values. Therefore, consumers
   // must not assume that the OccupancyStatus values follow a linear scale.
-  // Consumers should represent OccupancyStatus values as the state indicated
+  // Consumers should represent OccupancyStatus values as the state indicated 
   // and intended by the producer. Likewise, producers must use OccupancyStatus values that
   // correspond to actual vehicle occupancy states.
   // For describing passenger occupancy levels on a linear scale, see `occupancy_percentage`.
@@ -504,7 +545,7 @@ message VehiclePosition {
   // including both seated and standing capacity, and current operating regulations allow.
   // The value may exceed 100 if there are more passengers than the maximum designed capacity.
   // The precision of occupancy_percentage should be low enough that individual passengers cannot be tracked boarding or alighting the vehicle.
-  // If multi_carriage_status is populated with per-carriage occupancy_percentage,
+  // If multi_carriage_status is populated with per-carriage occupancy_percentage, 
   // then this field should describe the entire vehicle with all carriages accepting passengers considered.
   // This field is still experimental, and subject to change. It may be formally adopted in the future.
   optional uint32 occupancy_percentage = 10;
@@ -539,7 +580,7 @@ message VehiclePosition {
     // For example, the first carriage in the direction of travel has a value of 1.
     // If the second carriage in the direction of travel has a value of 3,
     // consumers will discard data for all carriages (i.e., the multi_carriage_details field).
-    // Carriages without data must be represented with a valid carriage_sequence number and the fields
+    // Carriages without data must be represented with a valid carriage_sequence number and the fields 
     // without data should be omitted (alternately, those fields could also be included and set to the "no data" values).
     // This message/field is still experimental, and subject to change. It may be formally adopted in the future.
     optional uint32 carriage_sequence = 5;
@@ -554,12 +595,12 @@ message VehiclePosition {
   }
 
   // Details of the multiple carriages of this given vehicle.
-  // The first occurrence represents the first carriage of the vehicle,
-  // given the current direction of travel.
-  // The number of occurrences of the multi_carriage_details
+  // The first occurrence represents the first carriage of the vehicle, 
+  // given the current direction of travel. 
+  // The number of occurrences of the multi_carriage_details 
   // field represents the number of carriages of the vehicle.
-  // It also includes non boardable carriages,
-  // like engines, maintenance carriages, etc… as they provide valuable
+  // It also includes non boardable carriages, 
+  // like engines, maintenance carriages, etc… as they provide valuable 
   // information to passengers about where to stand on a platform.
   // This message/field is still experimental, and subject to change. It may be formally adopted in the future.
   repeated CarriageDetails multi_carriage_details = 11;
@@ -648,7 +689,7 @@ message Alert {
   optional SeverityLevel severity_level = 14 [default = UNKNOWN_SEVERITY];
 
   // TranslatedImage to be displayed along the alert text. Used to explain visually the alert effect of a detour, station closure, etc. The image must enhance the understanding of the alert. Any essential information communicated within the image must also be contained in the alert text.
-  // The following types of images are discouraged : image containing mainly text, marketing or branded images that add no additional information.
+  // The following types of images are discouraged : image containing mainly text, marketing or branded images that add no additional information. 
   // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
   optional TranslatedImage image = 15;
 
@@ -791,14 +832,10 @@ message TripDescriptor {
     // enough to the scheduled trip to be associated with it.
     SCHEDULED = 0;
 
-    // An extra trip that was added in addition to a running schedule, for
-    // example, to replace a broken vehicle or to respond to sudden passenger
-    // load.
-    // NOTE: Currently, behavior is unspecified for feeds that use this mode. There are discussions on the GTFS GitHub
-    // [(1)](https://github.com/google/transit/issues/106) [(2)](https://github.com/google/transit/pull/221)
-    // [(3)](https://github.com/google/transit/pull/219) around fully specifying or deprecating ADDED trips and the
-    // documentation will be updated when those discussions are finalized.
-    ADDED = 1;
+    // This value has been deprecated as the behavior was unspecified. 
+    // Use DUPLICATED for an extra trip that is the same as a scheduled trip except the start date or time, 
+    // or NEW for an extra trip that is unrelated to an existing trip.
+    ADDED = 1 [deprecated = true];
 
     // A trip that is running with no schedule associated to it (GTFS frequencies.txt exact_times=0).
     // Trips with ScheduleRelationship=UNSCHEDULED must also set all StopTimeUpdates.ScheduleRelationship=UNSCHEDULED.
@@ -807,8 +844,8 @@ message TripDescriptor {
     // A trip that existed in the schedule but was removed.
     CANCELED = 3;
 
-    // Should not be used - for backwards-compatibility only.
-    REPLACEMENT = 5 [deprecated = true];
+    // A trip that replaces an existing trip in the schedule.
+    REPLACEMENT = 5;
 
     // An extra trip that was added in addition to a running schedule, for example, to replace a broken vehicle or to
     // respond to sudden passenger load. Used with TripUpdate.TripProperties.trip_id, TripUpdate.TripProperties.start_date,
@@ -816,8 +853,9 @@ message TripDescriptor {
     // date and/or time. Duplicating a trip is allowed if the service related to the original trip in (CSV) GTFS
     // (in calendar.txt or calendar_dates.txt) is operating within the next 30 days. The trip to be duplicated is
     // identified via TripUpdate.TripDescriptor.trip_id. This enumeration does not modify the existing trip referenced by
-    // TripUpdate.TripDescriptor.trip_id - if a producer wants to cancel the original trip, it must publish a separate
-    // TripUpdate with the value of CANCELED or DELETED. Trips defined in GTFS frequencies.txt with exact_times that is
+    // TripUpdate.TripDescriptor.trip_id - if a producer wants to replace the original trip, a value of `REPLACEMENT` should be used instead.
+    //
+    // Trips defined in GTFS frequencies.txt with exact_times that is
     // empty or equal to 0 cannot be duplicated. The VehiclePosition.TripDescriptor.trip_id for the new trip must contain
     // the matching value from TripUpdate.TripProperties.trip_id and VehiclePosition.TripDescriptor.ScheduleRelationship
     // must also be set to DUPLICATED.
@@ -837,9 +875,39 @@ message TripDescriptor {
     // real-time predictions.
     // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
     DELETED = 7;
+
+    // An extra trip unrelated to any existing trips, for example, to respond to sudden passenger load.
+    // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+    NEW = 8;
   }
   optional ScheduleRelationship schedule_relationship = 4;
 
+  message ModifiedTripSelector {
+    // The 'id' from the FeedEntity in which the contained TripModifications object affects this trip.
+    optional string modifications_id = 1;
+
+    // The trip_id from the GTFS feed that is modified by the modifications_id
+    optional string affected_trip_id = 2;
+
+    // The initially scheduled start time of this trip instance, applied to the frequency based modified trip. Same definition as start_time in TripDescriptor.
+    optional string start_time = 3;
+
+    // The start date of this trip instance in YYYYMMDD format, applied to the modified trip. Same definition as start_date in TripDescriptor.
+    optional string start_date = 4;
+
+    // The extensions namespace allows 3rd-party developers to extend the
+    // GTFS Realtime Specification in order to add and evaluate new features and
+    // modifications to the spec.
+    extensions 1000 to 1999;
+
+    // The following extension IDs are reserved for private use by any organization.
+    extensions 9000 to 9999;
+  }
+
+  // Linkage to any modifications done to this trip (shape changes, removal or addition of stops).
+  // If this field is provided, the `trip_id`, `route_id`, `direction_id`, `start_time`, `start_date` fields of the `TripDescriptor` MUST be left empty, to avoid confusion by consumers that aren't looking for the `ModifiedTripSelector` value.
+  optional ModifiedTripSelector modified_trip = 7;
+
   // The extensions namespace allows 3rd-party developers to extend the
   // GTFS Realtime Specification in order to add and evaluate new features and
   // modifications to the spec.
@@ -970,12 +1038,12 @@ message TranslatedString {
 message TranslatedImage {
   message LocalizedImage {
     // String containing an URL linking to an image
-    // The image linked must be less than 2MB.
+    // The image linked must be less than 2MB. 
     // If an image changes in a significant enough way that an update is required on the consumer side, the producer must update the URL to a new one.
-    // The URL should be a fully qualified URL that includes http:// or https://, and any special characters in the URL must be correctly escaped. See the following http://www.w3.org/Addressing/URL/4_URI_Recommentations.html for a description of how to create fully qualified URL values.
+    // The URL should be a fully qualified URL that includes http:// or https://, and any special characters in the URL must be correctly escaped. See the following http://www.w3.org/Addressing/URL/4_URI_Recommentations.html for a description of how to create fully qualified URL values.
     required string url = 1;
 
-    // IANA media type as to specify the type of image to be displayed.
+    // IANA media type as to specify the type of image to be displayed. 
     // The type must start with "image/"
     required string media_type = 2;
 
@@ -1033,3 +1101,148 @@ message Shape {
   // The following extension IDs are reserved for private use by any organization.
   extensions 9000 to 9999;
 }
+
+// Describes a stop which is served by trips. All fields are as described in the GTFS-Static specification.
+// NOTE: This message is still experimental, and subject to change. It may be formally adopted in the future.
+message Stop {
+  enum WheelchairBoarding {
+    UNKNOWN = 0;
+    AVAILABLE = 1;
+    NOT_AVAILABLE = 2;
+  }
+
+  optional string stop_id = 1;
+  optional TranslatedString stop_code = 2;
+  optional TranslatedString stop_name = 3;
+  optional TranslatedString tts_stop_name = 4;
+  optional TranslatedString stop_desc = 5;
+  optional float stop_lat = 6;
+  optional float stop_lon = 7;
+  optional string zone_id = 8;
+  optional TranslatedString stop_url = 9;
+  optional string parent_station = 11;
+  optional string stop_timezone = 12;
+  optional WheelchairBoarding wheelchair_boarding = 13 [default = UNKNOWN];
+  optional string level_id = 14;
+  optional TranslatedString platform_code = 15;
+
+  // The extensions namespace allows 3rd-party developers to extend the
+  // GTFS Realtime Specification in order to add and evaluate new features and
+  // modifications to the spec.
+  extensions 1000 to 1999;
+
+  // The following extension IDs are reserved for private use by any organization.
+  extensions 9000 to 9999;
+}
+
+// NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+message TripModifications {
+  // A `Modification` message replaces a span of n stop times from each affected trip starting at `start_stop_selector`.
+  message Modification {
+    // The stop selector of the first stop_time of the original trip that is to be affected by this modification.
+    // Used in conjuction with `end_stop_selector`. 
+    // `start_stop_selector` is required and is used to define the reference stop used with `travel_time_to_stop`.
+    optional StopSelector start_stop_selector = 1;
+
+    // The stop selector of the last stop of the original trip that is to be affected by this modification. 
+    // The selection is inclusive, so if only one stop_time is replaced by that modification, `start_stop_selector` and `end_stop_selector` must be equivalent.
+    // If no stop_time is replaced, `end_stop_selector` must not be provided. It's otherwise required.
+    optional StopSelector end_stop_selector = 2;
+
+    // The number of seconds of delay to add to all departure and arrival times following the end of this modification. 
+    // If multiple modifications apply to the same trip, the delays accumulate as the trip advances. 
+    optional int32 propagated_modification_delay = 3 [default = 0];
+
+    // A list of replacement stops, replacing those of the original trip. 
+    // The length of the new stop times may be less, the same, or greater than the number of replaced stop times. 
+    repeated ReplacementStop replacement_stops = 4;
+
+    // An `id` value from the `FeedEntity` message that contains the `Alert` describing this Modification for user-facing communication.
+    optional string service_alert_id = 5;
+
+    // This timestamp identifies the moment when the modification has last been changed.
+    // In POSIX time (i.e., number of seconds since January 1st 1970 00:00:00 UTC).
+    optional uint64 last_modified_time = 6;
+
+    // The extensions namespace allows 3rd-party developers to extend the
+    // GTFS Realtime Specification in order to add and evaluate new features and
+    // modifications to the spec.
+    extensions 1000 to 1999;
+
+    // The following extension IDs are reserved for private use by any organization.
+    extensions 9000 to 9999;
+  }
+
+  message SelectedTrips {
+    // A list of trips affected with this replacement that all have the same new `shape_id`. A `TripUpdate` with `schedule_relationship=REPLACEMENT` must not already exist for the trip.
+    repeated string trip_ids = 1;
+    // The ID of the new shape for the modified trips in this SelectedTrips.
+    // May refer to a new shape added using a GTFS-RT Shape message, or to an existing shape defined in the GTFS-Static feed’s shapes.txt.
+    optional string shape_id = 2;
+
+    // The extensions namespace allows 3rd-party developers to extend the
+    // GTFS Realtime Specification in order to add and evaluate new features and
+    // modifications to the spec.
+    extensions 1000 to 1999;
+
+    // The following extension IDs are reserved for private use by any organization.
+    extensions 9000 to 9999;
+  }
+
+  // A list of selected trips affected by this TripModifications.
+  repeated SelectedTrips selected_trips = 1;
+
+  // A list of start times in the real-time trip descriptor for the trip_id defined in trip_ids. 
+  // Useful to target multiple departures of a trip_id in a frequency-based trip.
+  repeated string start_times = 2;
+
+  // Dates on which the modifications occurs, in the YYYYMMDD format. Producers SHOULD only transmit detours occurring within the next week.
+  // The dates provided should not be used as user-facing information, if a user-facing start and end date needs to be provided, they can be provided in the linked service alert with `service_alert_id`
+  repeated string service_dates = 3;
+
+  // A list of modifications to apply to the affected trips. 
+  repeated Modification modifications = 4;
+
+  // The extensions namespace allows 3rd-party developers to extend the
+  // GTFS Realtime Specification in order to add and evaluate new features and
+  // modifications to the spec.
+  extensions 1000 to 1999;
+
+  // The following extension IDs are reserved for private use by any organization.
+  extensions 9000 to 9999;
+}
+
+// NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+// Select a stop by stop sequence or by stop_id. At least one of the two values must be provided.
+message StopSelector {
+  // Must be the same as in stop_times.txt in the corresponding GTFS feed.
+  optional uint32 stop_sequence = 1;
+  // Must be the same as in stops.txt in the corresponding GTFS feed.
+  optional string stop_id = 2;
+
+  // The extensions namespace allows 3rd-party developers to extend the
+  // GTFS Realtime Specification in order to add and evaluate new features and
+  // modifications to the spec.
+  extensions 1000 to 1999;
+
+  // The following extension IDs are reserved for private use by any organization.
+  extensions 9000 to 9999;
+}
+
+// NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
+message ReplacementStop {
+  // The difference in seconds between the arrival time at this stop and the arrival time at the reference stop. The reference stop is the stop prior to start_stop_selector. If the modification begins at the first stop of the trip, then the first stop of the trip is the reference stop.
+  // This value MUST be monotonically increasing and may only be a negative number if the first stop of the original trip is the reference stop.
+  optional int32 travel_time_to_stop = 1;
+
+  // The replacement stop ID which will now be visited by the trip. May refer to a new stop added using a GTFS-RT Stop message, or to an existing stop defined in the GTFS-Static feed’s stops.txt. The stop MUST have location_type=0 (routable stops).
+  optional string stop_id = 2;
+
+  // The extensions namespace allows 3rd-party developers to extend the
+  // GTFS Realtime Specification in order to add and evaluate new features and
+  // modifications to the spec.
+  extensions 1000 to 1999;
+
+  // The following extension IDs are reserved for private use by any organization.
+  extensions 9000 to 9999;
+}
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