That was a problematic design by Lionel. There should be a nylon and steel washer between the commutator and brush plate to limit vertical movement. But the main problem is shifting of the bottom bearing plate in the frame that causes the armature to be drawn into the field when going in one direction that causes it to bind. Lionel’s fix was to stake the sides of the frame to lock the bearing plate in place. I’ve had limited success with this fix. It’s a trial and error to find the best location of bearing plate.
Lionel improved this design by eliminating the bottom bearing plate and installing the motor bearing in the upper frame.
