Enhancing GTFS Schedule and Realtime with original_trip_id

gtfs-realtime/proto/gtfs-realtime.proto

@@ -873,6 +873,10 @@ message TripDescriptor {
   // 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;
 
+  // Uniquely identifies one or more trips per operating day and across different days of a scheduled year (see also GTFS Schedule definition).
+  // It allows to compare and match GTFS-trips with similar constructs from other standards (e.g. NeTEx/SIRI) or with schedule. GTFS Realtime data can even be used without GTFS Schedule data.
+  optional string original_trip_id = 8;
+
   // 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.

gtfs-realtime/spec/en/reference.md

@@ -466,6 +466,7 @@ TripDescriptor.route_id cannot be used within an Alert EntitySelector to specify
 | **start_date** | [string](https://protobuf.dev/programming-guides/proto2/#scalar) | Conditionally required | One | The start date of this trip instance in YYYYMMDD format. For scheduled trips (trips not defined in GTFS frequencies.txt), this field must be provided to disambiguate trips that are so late as to collide with a scheduled trip on a next day. For example, for a train that departs 8:00 and 20:00 every day, and is 12 hours late, there would be two distinct trips on the same time. This field can be provided but is not mandatory for schedules in which such collisions are impossible - for example, a service running on hourly schedule where a vehicle that is one hour late is not considered to be related to schedule anymore. This field is required for frequency-based trips defined in GTFS frequencies.txt. If trip_id is omitted, start_date must be provided. |
 | **schedule_relationship** | [ScheduleRelationship](#enum-schedulerelationship-1) | Optional | One | The relation between this trip and the static schedule. If TripDescriptor is provided in an Alert `EntitySelector`, the `schedule_relationship` field is ignored by consumers when identifying the matching trip instance.
 | **modified_trip** | [ModifiedTripSelector](#message-modifiedtripselector) | Optional | One | 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. 
+| **original_trip_id** | [string](https://protobuf.dev/programming-guides/proto2/#scalar) | Optional | One | Uniquely identifies one or more trips per operating day and across different days of a scheduled year (see also GTFS Schedule definition). It allows to compare and match GTFS-trips with similar constructs from other standards (e.g. NeTEx/SIRI) or with schedule. GTFS Realtime data can even be used without GTFS Schedule data. |
 
 ## _enum_ ScheduleRelationship
 

gtfs/spec/en/reference.md

@@ -261,6 +261,7 @@ Primary key (`trip_id`)
 |  `shape_id` | Foreign ID referencing `shapes.shape_id` | **Conditionally Required** | Identifies a geospatial shape describing the vehicle travel path for a trip. <br><br>Conditionally Required: <br>- **Required** if the trip has a continuous pickup or drop-off behavior defined either in [routes.txt](#routestxt) or in [stop_times.txt](#stop_timestxt). <br>- Optional otherwise. |
 |  `wheelchair_accessible` | Enum | Optional | Indicates wheelchair accessibility. Valid options are:<br><br>`0` or empty - No accessibility information for the trip.<br>`1` - Vehicle being used on this particular trip can accommodate at least one rider in a wheelchair.<br>`2` - No riders in wheelchairs can be accommodated on this trip. |
 |  `bikes_allowed` | Enum | Optional | Indicates whether bikes are allowed. Valid options are:<br><br>`0` or empty - No bike information for the trip.<br>`1` - Vehicle being used on this particular trip can accommodate at least one bicycle.<br>`2` - No bicycles are allowed on this trip. |
+| `original_trip_id` | ID | Optional | Identifies a logical composition of one or more trips. Unique per operating day, which can be longer than 24hrs. Can reoccur across different days of a scheduled year. Can be used to compare and match GTFS trips with similar constructs from other standards (e.g. SIRI/NeTEx) or with realtime. For example, two trips that operate the same sequence of stops on Saturdays and Sundays but deviate by 1 minute in their arrival time at the final stop. Those trips can have the same original_trip_id, but not the same trip_id. |
 
 #### Example: Blocks and service day