How Live Train Tracking Works — And Why Only Some Trains Are Live

Open a live flight tracker and the whole sky is full of moving planes. Open a live ship map and the oceans crawl with vessels. Do the same for trains and something strange happens: a few cities light up with moving dots, and the rest of the world's vast rail network sits perfectly still. That isn't a gap in the map. It's the honest state of the world — and it comes down to how each kind of vehicle tells the world where it is.

Planes and ships broadcast; trains mostly don't

The reason you can watch nearly every airliner on Earth is a small radio box on each aircraft called an ADS-B transponder. It broadcasts the plane's GPS position constantly, in the open, and anyone with a cheap receiver can hear it. Thousands of volunteers do exactly that and pool the data, which is why global live flight maps exist at all. Ships have their own version, AIS, doing the same job over the water.

Trains have no equivalent. There is no open signal a train broadcasts that a hobbyist can pick up from a rooftop. A train's position is known only inside its operator's own systems — and whether that position ever reaches the public depends entirely on whether the operator decides to publish it.

The feed that makes a live train possible

The transit agencies that do publish use a shared format called GTFS-Realtime. It has two relevant parts, and the difference between them is the whole story:

• VehiclePositions — the real thing. A live GPS fix for each train: where it physically is, right now. This is what a map needs to draw a dot in the right place.
• TripUpdates — predictions. Estimated arrival times at upcoming stops. Useful for "next train in 4 minutes," but it is not a position. You cannot honestly place a train on a map from a TripUpdate.

An agency might publish both, one, or neither. This is why coverage is so uneven. The New York City subway, for instance, publishes detailed TripUpdates but not vehicle positions for most lines — so its trains can power an arrivals board but genuinely cannot be placed on a live map. We leave them off rather than fake a location from an arrival estimate.

Why the live layer lights up only a few cities

Put those two facts together and the patchwork explains itself. A system appears on the live map only if its operator publishes real VehiclePositions with a feed that is free to use. Relatively few do. Passenger and transit agencies are the main publishers; freight railroads, which run most of the track kilometres in a country like the United States, keep their train positions private almost everywhere.

So the honest picture is a handful of metros glowing against a dark network. Today that means Boston — the MBTA's subway lines and commuter rail — and New York — the Long Island Rail Road and Metro-North. Each dot there is a real train reporting its own position. Everywhere else, there is simply no live feed to draw from, and the map says so.

How we draw them, and how we stay honest

When you switch the Live Trains layer on, we fetch each published feed, decode it on our own servers, and draw only the processed positions. A few rules keep it truthful:

• A dot is only ever a real reported position. If a feed omits a train's location, that train doesn't appear — we never interpolate a position or invent one.
• We show the freshness. Each system's feed carries a timestamp, and we display how long ago it last updated, so a stale feed is visible instead of masquerading as live.
• No stale-as-live fallback. Trains move fast, so if a feed fails we don't keep showing its last known dots under a "live" badge. Empty and honest beats stale and wrong.
• A quiet hour is a quiet hour. Overnight, a system may report very few trains or none. That's the feed reflecting reality, not a fault.

The tracks themselves are a separate, static layer — a dated snapshot of where the world's railways run. The illustrative Trains layer sends representative trains gliding along the great corridors purely for atmosphere, clearly labelled as not live. And the Live Trains layer shows the genuine article, for the few systems that make it possible. Three different things, each labelled for exactly what it is — because the interesting truth here isn't "look, live trains everywhere," it's why live trains aren't everywhere, and where they honestly are.