Unless you're providing the motor control, I think measuring the back-EMF may be somewhat difficult, you have to time it for when the motor is not getting drive voltage. FWIW, while the motor voltage is not an iron-clad indicator, it's a ton easier to use. Also, if the locomotive has cruise, voltage on the motor will certainly indicate it's moving. It really depends on the actual locomotive you're trying to add the feature to.
I agree with Stan, the four locations should be independent and random.
As for curves, the Lionel Legacy Vision Line Tank Cars from 2009 had an angle sensor to change the sounds on curves, it was a simple arrangement. It had a hanging pendulum that swung out on curves and a sensor board under it that triggered the sound board to recognize the curves. It works great on the tank cars, I love the effect and the reaction to curves, really adds a dimension to the sound generation.
Here's some of the pieces used in the tank car.
