These little tags that are passive until excited by a reading antenna can not only tell you that a train is at the location, it also tells you which train it is.
It looks like a company called Littfinski DatenTechnik is leading the way with a small range of RFID products.
While this technology looks pretty interesting, it is still in early stages.
And at 4 Euro per ID and 90 Euro per detector, it is not cheap.
Still something worth keeping an eye on I feel!