Realtime GTFS files
The GTFS Realtime standard uses the protobuf format to transfer compact messages which describe deviations between the real world and the schedules for a public transport network. There are 3 different feeds, each of which provides their own type of information.
ServiceAlerts provide information about incidents, planned maintenance, … which affect more than one or a couple of trips. They can describe a closed station, incidents like electricity failures or defect trains, extra services for certain events, … . They typically contain similar information as the messages shown on digital signs in stations. Usually a header, description and link are provided, along with a cause and an effect (provided as an enum). Service alerts can be limited in time or to certain agencies, trips, stops, transport types, …
TripUpdates provide information about deviations between the scheduled timetable of a trip and the real world. This can be a delay, a vehicle running early, a partial cancellation, a platform change, … . Each TripUpdates file contains multiple TripDescriptors, each of which describes deviations in a trip. If a scheduled trip isn’t present in a TripUpdates file, this can both mean that there are no delays, or that no information is available.
VehicleUpdates contain realtime GPS positions of public transport vehicles. These allow you to see where a vehicle is located, along with its speed and bearing. These can be used to show vehicles on a map, to analyze traffic congestion, …