Why a train’s dot can drift off course
A phone does not receive a live report from the train. It estimates its own location by measuring the timing of signals from several navigation satellites. Because those signals travel at the speed of light, tiny timing differences help the phone calculate where it is. The app then places that estimate on a map and updates it as the phone moves.
That distinction matters on rail journeys: the dot represents the phone’s best estimate, not the train’s official position or progress along the track. If the estimate is imprecise, the map may show the phone beside the railway or on a nearby road.
What changes inside a tunnel
Satellite signals are weak by the time they reach Earth, and rock, concrete, and metal can block them. In a tunnel, the phone may lose enough signals that it can no longer make a fresh, reliable location fix. Its dot may freeze, disappear, or continue moving briefly based on an estimate rather than a new satellite reading.
Some phones can also use motion sensors—such as accelerometers and gyroscopes—to estimate movement between location fixes. These sensors can help bridge a short gap, but small errors accumulate because they are measuring motion without a dependable starting point. Once the phone gets a clear satellite signal again, the app can correct its position, sometimes making the dot jump.
The map is not the railway’s control system
A route drawn on a map can make the dot look like a live train tracker, but it is only a visual comparison between the phone’s estimated location and mapped lines. Railway operators use separate systems to monitor and manage trains. For a passenger, the practical consequence is simple: a briefly stalled dot in a tunnel says more about the phone’s lost signal than about whether the train has stopped.
