What "kills" a locomotive PC board?

[torikoos]

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.

In that case, I would suspect that your carbon brushes are worn/damaged and may have moved so that they are jamming against the commutator of the motor, and the brush holder. The commutator has small 'gaps' which spinning in one direction may pass the brush smoothly, but in the other direction perhaps cause the brush to jam it somewhat. If you can turn the motor by hand, you may be able to feel or hear this, as one direction it will be smooth (ish), the other you may feel it jamming up or hear it ticking when you turn it.

In any case, take out the brushes, clean out the motor with compressed air ( air from an airbrush will work for example), and before re-installing (new) brushes, feel if it rotates ok in both directions. Look for physical damage. If all is well , a few drops of lube on the motor 'bearings' and you can try again.

Actually re-reading your post again, you wrote there's no problem with motor spinning freely (unless the offending 'brush' perhaps has already shifted due to the handling. There might now not be a brush that is properly seated, a slight movement will cause it to loose connection one way, and 'just' the other direction. In any case, check it all over. If you can get the motor to run ok in both directions on DC (with a pair of wires attached to a power pack) then it should be fine for DCC too. I left the above 'jam' scenario for info purposes.

Koos