We would like to remind developers of the use of the TripDescriptor.NyctTripDescriptor.is_assigned field in NYCT’s subway GTFS-realtime feeds. As described in our GTFS-realtime Reference for the New York City Subway, the field is used as follows:
Train Assignment is a function of the Automatic Train Supervision (ATS) office system used by NYCT Rail Operations to monitor and track train movements. ATS provides the ability to “assign” the nyct_train_id attribute when a physical train is at its origin terminal. These assigned trips have the is_assigned field set in the TripDescriptor. When a train is at a terminal but has not been given a work program it is declared unassigned and is tagged as such. Unassigned trains can be moved to a storage location or assigned a nyct_train_id when a determination for service is made.
In other words, it is normal and expected for trips which are still at their origin terminal to have their is_assigned attribute set to false. However, once a trip’s origin departure time (that is, the departure time of the first StopTimeUpdate) has passed, if the trip is still unassigned (that is, the TripDescriptor.is_assigned attribute remains false), that is a strong signal that there may not be a physical train associated with the trip. Developers of passenger-facing applications may wish to identify these trips in some way (that is, to flag that the trip may not be operating), or suppress the arrival entirely.
NYCT | Subways | Technology
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The short answer is “no”, unfortunately – this is an element of how subway bookkeeping works. Whenever a trip is re-routed, short-turned, directed to skip stops or run express, etc., the train’s “call letters” (what is in the nyct_train_id field) will change. Because of how the GTFS trip_id is derived, it may also change if the path changes during the course of the trip (that is, if the train is re-routed).
NYCT | Subways | Technology
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I can’t guarantee that this will hold true in all operating scenarios – but yes, there are situations where the origin time, route, and direction of travel will remain constant but the path will change to a new value.
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