Train Describer
Also: td, td feed, describer
The part of the signalling that shows each train’s headcode moving along the signaller’s screen.
Signals detect where trains are. The Train Describer keeps track of which train is which, carrying each headcode from berth to berth as the train moves, so the signaller always knows what they’re looking at.
Network Rail publishes every one of these moves, across the country, seconds after it happens. That’s the most detailed live view of trains the open data offers.
In Trainiac
The live map leans on it most of all.