This Live Train Map of the London Underground tracks every train on the network in real time and is awesome. Watching the little dots moving around the map is entrancing, and pretty useful if you’re expecting to catch one of them in 2 minutes time. Thank you Mr Somverville (and for your national rail network version).
This map shows all trains currently on approach to a particular station. The yellow pins are stations (click for a local map of that station), the red pins trains. The trains move in approximately real time, or rather quicker if you check the speed-up box.