diff --git a/gtfs-realtime/spec/en/reference.md b/gtfs-realtime/spec/en/reference.md
index a8a4ae086..6089b9581 100644
--- a/gtfs-realtime/spec/en/reference.md
+++ b/gtfs-realtime/spec/en/reference.md
@@ -264,6 +264,7 @@ Defines updated properties of the trip.
 | **shape_id** | [string](https://protobuf.dev/programming-guides/proto2/#scalar) | Optional | One | Specifies the identifier of the shape of the vehicle travel path when the trip shape differs from the shape specified in (CSV) GTFS or to specify it in real-time when it's not provided by (CSV) GTFS, such as a vehicle that takes differing paths based on rider demand. See definition of `trips.shape_id` in (CSV) GTFS. <br>If a shape is neither defined in (CSV) GTFS nor in real-time, the shape is considered unknown. This field can refer to a shape defined in the (CSV) GTFS in shapes.txt or a `Shape` in the same (protobuf) real-time feed. The order of stops (stop sequences) for this trip must remain the same as (CSV) GTFS. If it refers to a `Shape` entity in the same real-time feed, the value of this field should be the one of the `shape_id` inside the entity, and _not_ the `id` of `FeedEntity`.<br>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 or more details can be provided via a `TripModifications` message.<br><br>**Caution:** this field is still **experimental**, and subject to change. It may be formally adopted in the future. |
 | **trip_headsign** | [string](https://protobuf.dev/programming-guides/proto2/#scalar) | Optional | One | Specifies the headsign for this trip when it differs from the original. <br><br>**Caution:** this field is still **experimental**, and subject to change. It may be formally adopted in the future. |
 | **trip_short_name** | [string](https://protobuf.dev/programming-guides/proto2/#scalar) | Optional | One | Specifies the name for this trip when it differs from the original. <br><br>**Caution:** this field is still **experimental**, and subject to change. It may be formally adopted in the future. |
+| **route_id** | [string](https://protobuf.dev/programming-guides/proto2/#scalar) | Optional | One | Specifies the route associated with the trip, if the route has been added dynamically. The associated `Route` message should be included in the same feed. |
 
 ## _message_ VehiclePosition