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 sens

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.