The problem is almost certainly play in the black plastic plate at the bottom of the motor truck. It acts as the lower armature bearing. This is a well known problem with those motors.
To figure out which way the plate has to go, try to run the motor in the "bad" direction. With power still applied, turn the engine over and push the plate to one side, and then the other. It may only move a tiny bit, but in one direction the motor will take off.
There are various ways to make things tighter so the plate stays in place. The Lionel service manual suggested using a center punch to dimple the metal side frame. I have wedged a paper matchstick between the metal side frame and the plastic plate. A drop of CA, and the matchstick isn't going anywhere.
I am certain other people will add their solutions.
