Without seeing it, I can't say if it's the motor or gears. When the motors were on the bench, did you do any tests lightly holding the flywheel to put a load on them to see if they seemed to have plenty of torque?
Did you try connecting the DC power supply to the motors when they're installed in the locomotive? Just pull the plug from the board and connect to one motor with the DC supply. Also, testing them individually while actually connected to the trucks with the DC supply should be telling.