GTFS-Realtime
The network, right now.
GTFS-Realtime is the live companion to the timetable. It says where vehicles are and how the schedule is holding up.
Three kinds of message.
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Vehicle positions
Where each vehicle is at this moment, drawn from vehicle location.
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Trip updates
Predicted arrival and departure times, including delays and cancellations.
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Service alerts
Detours, closures and other disruptions riders should know about.
Messages are encoded with Protocol Buffers, a compact binary format, and each one points back to a trip or stop in the GTFS timetable.
In TransitAI Core it is the contract for live data: what comes down the streaming lane is shaped to match it.