Developer portal · Glossary
The domain terms that show up across the API and these docs, in plain language.
The operating day used for all historical data, running 03:00–02:59 (Europe/London) rather than midnight to midnight. A date like 2026-07-07 refers to that boundary, so trains running after midnight still belong to the previous calendar date's railway day.
A single pre-built, enriched picture of a line at a moment in time — the JSON a live endpoint returns. Snapshots are rebuilt roughly every 30 seconds and archived for replay.
The map snapshot is a superset covering everything shown on the app's map for a line, including adjacent services. The line snapshot is just that one line, including its terminals. Only the map flavour is archived for replay.
TfL's TrackerNet feed is queried per station, not per train. A train approaching appears in several nearby stations' countdown lists at once; those sightings are de-duplicated into one record. See data sources.
currentLocation is the train's true reported position (e.g. "Between Baker Street and Finchley Road"). polledStationName is merely the station whose countdown produced the winning sighting — it is not reliably the next stop, so position logic anchors on currentLocation.
A train that stops reporting movement for long enough is flagged with isMaybeStalled, along with stallDurationSeconds and stallSince. This stops downstream ETAs and boards from silently going stale on a train that isn't actually moving.
When a train changes its identity mid-service (for example, being renumbered), Trackser Live tracks the change with isReformed and oldTrainNumber so its journey stays continuous.
The physical identifier of the rolling stock working a service — leadingCarNo and rearCarNo — as distinct from the operational train number. A unit is that physical vehicle, tracked across the diagrams it works.
How a train calls along its route — All Stations, Semi Fast, Fast, via variants, or a Booked: … pattern taken from the working timetable (primarily on the Metropolitan line).
The operational schedule of booked trips, platforms and stopping patterns. Trackser Live overlays WTT data onto live trains where available — currently for the Metropolitan and Chiltern lines.
On a station board, secondsAway is an estimated live ETA rebased onto the train's true position, and stopsAway is a topological count of intermediate stops. Either can be null when the data is missing or the branch is ambiguous.
If a live snapshot hasn't been built yet, the endpoint returns 202, triggers a build, and asks the client to retry shortly — rather than returning stale or empty data.
Each line's live state and snapshot-building is owned by a single Cloudflare Durable Object instance, keyed by line ID — so lines are isolated from one another. See architecture.