[captain_price7]

Sorry, but this seems too strange to be true. Are you sure you didn't miss anything?

Particularly strange since moving train (i.e. vehicle) is about the most common way doppler effect is explained in textbooks- it's not like you need any big "eureka" moment to get to this solution either.

[bostonpete]

Analyzing the doppler shift to calculate speed only works if you know what the unshifted audio spectrum should be. Trains generate a ton of noise at a wide range of frequencies and that noise probably varies significantly based on a bunch of factors.

[93po]

If you put the microphone directly against the track, I would bet the friction and movement of the wheels against the track generates vibration that is fairly consistent for a given speed. Maybe a sensor that better detects slight vibration would be better than a microphone for this use case.

Additionally, train engines run as generators to actually power the wheels, which means they're likely running at consistent RPMs or a consistent range of set RPMs. This could be listened for.