What "kills" a locomotive PC board?

[Green_Elite_Cab]

Did something go wrong in the gear towers of one of the trucks? Is there dirt inside perhaps?
It could be that dirt or other malfunction makes the gears run with more resistance in one direction, and not the other. This could perhaps have overloaded the motor and fry it?
It is very hard to diagnose 'remotely', especially without any pictures and perhaps other information. At first, I'm calling a mechanical issue being at the root of this, as everything happened in 'reverse' direction, and not the other...

I might have gone for the mechanical explanation, but I was able to freely turn the drive by hand.

Furthermore, the motor will not run in reverse even when disconnected from the drive (as in, spinning freely). It turns freely by hand, but not under power.