On our Christmas layout we were running the lionchief Polar Express set that stopped running. The motor would spin but the engine would not move.
This is what I found.
The motor was only being held in with one screw not two. This caused the motor to move which wore out the worm gear on the motor shaft. There NEVER was a second screw installed for the motor mounting as the hole for it was filled in with grease.
Before the failure the engine was running fine there was no warning that there was a problem. This is a big disappointment for us. We have a very large number of children come through at this time of year during their holiday break. Happening this close to Christmas we are unable to get a replacement motor until next Tuesday.
This may be something that only we experienced BUT
You may want to pull the bottom plate off your engine to make sure both motor mounting screws are there. This is a simple check and will save you the problem we now have.
