On my local tram network you could even do that just by listening to the noises made by the inverters, which drive the motors. They have a very distinct hum th

On my local tram network you could even do that just by listening to the noises made by the inverters, which drive the motors. They have a very distinct hum that matches the rpm of the wheels. I once built a crude speedometer using SFTs, peak detection and kalman filtering.