The routing API backend is connected to vehicle real-time information for some routes. This means that API queries can return real-time data for some vehicles.
Basically real-time means prediction of arrival times. We know vehicle's planned route and it's current location. This information is used to predict when it actually arrives.
Real-time estimates are enabled by default when making itinerary planning requests. Availability of real-time estimates is not guaranteed, but field
realTime (true / false) in each leg of the itinerary will indicate whether that leg's arrival / departure time is using real-time or static timetables. When using real-time, it means that uncertainty for arrival time is less than for purely static timetable data.
Real-time estimates for itinerary planning can be turned off by using argument
ignoreRealtimeUpdates of plan query. Setting this to true will only return times based on static timetables.
For stop timetables and route timetables, real-time estimates are available from fields
realtimeDeparture. Note that these fields will have the static arrival / departure time by default and field
realtime indicates whether the values of those fields have been updated from a real-time source.
Note: Real-time information is not completely bullet proof yet. The quality and availability of the real-time data that Digitransit uses varies heavily at the moment.