Radar exists too.

The need might be for a sensor local to the platform as a back up to give warning for a train that's traveling too fast? In which case a sensor that mimics the old Cowboy film favourite of putting one's ear to the track seems like a reasonable thing to try.

Ah, we're talking about practical solutions here? Should've warned me. A chain of laser reflective sensors might be even better, because there is less mechanical wear + you can use them to know where the train currently is and where it isn't.

But this is very likely a very well researched area and there are definitly train people who can point out a flaw in this idea (dirt?)

Or some other type of sensor and minimal gear added to each locomotive...