Consume a GTFS-Realtime VehiclePositions feed for live vehicle location

Steps

1. Fetch the VehiclePositions protobuf feed URL published by the agency (often a separate endpoint from TripUpdates/Alerts)
2. Decode with the gtfs-realtime.proto VehiclePosition message per gtfs.org/documentation/realtime/reference/
3. Read position.latitude/longitude, and optionally bearing, odometer, and speed (meters/second)
4. Correlate vehicle.id or trip.trip_id back to the static feed to label the vehicle's route/headsign
5. Re-poll at the cadence recommended in the realtime best-practices doc since vehicle positions change fastest of the three feed types