A descriptor that identifies an instance of a GTFS trip, or all instances of
a trip along a route.
- To specify a single trip instance, the trip_id (and if necessary,
start_time) is set. If route_id is also set, then it should be same as one
that the given trip corresponds to.
- To specify all the trips along a given route, only the route_id should be
set. Note that if the trip_id is not known, then stop sequence ids in
TripUpdate are not sufficient, and stop_ids must be provided as well. In
addition, absolute arrival/departure times must be provided.