Give a train unit number — the number on the cab end, like 360107 — and this finds the service it is working now, everything else it worked today, and the last place it reported from. Individual coach numbers work too.
Unit numbers come from the industry's own allocation data, so they cover the operators that publish it. Several do not, and for those a unit will simply not be found — that is a gap in the data rather than a vehicle that is not running.
For trains, not very, and deliberately not pretended otherwise. Trains report as they pass timing points, so the position shown is the last point it was seen at rather than where it is now. Buses are different: they report their own position directly, so a bus pin is where the bus was when it last spoke.
The gap matters more than it sounds. On a fast line a train can be twenty miles beyond the last point it reported from, so the pin is better read as “it has passed here” than as “it is here”. Every response says which basis it is using rather than leaving a consumer to assume precision that does not exist.
A unit works all day. One Class 756 we checked was allocated to fifteen separate services on a single date, and nine to sixteen workings is entirely normal. So this page names the working it is on now — a service currently running, or failing that the last one to have departed — and lists everything else it worked that day underneath. A single pin without that context would imply the dot was the unit's whole day.
It also shows which days we hold an allocation for the number at all, with how many trains it worked on each. That is history for as long as the archive keeps it, and a missing day means no allocation was recorded rather than a unit standing idle.
Both are six digits and nothing about their shape separates them — 755403 is a whole unit, 911403 is one carriage inside it. Either works here,
because a person reading a number off the side of a train has no reason to know which
they are holding. A carriage is found through its formation, even though its number
appears nowhere on the unit's own record.
Three quite different things produce no result, and it is worth knowing which you are looking at:
The page says which of those applies rather than showing one blank for all three. What the formation data does and does not mean is covered at length in what a train is made of.