It may be impossible. The motor may have seized and caused armature damage. It ran fine until I noticed it was taking a long time back around the loop. One of the brushes gets very hot, there is a bit of vibration, and soon afterward, smoke. The armature turns freely by hand. Although this handcar was discontinued, unsure if Bachmann used the same motor in something else. And if they did, unsure if the armature shaft would be long enough for the worm gears (on each end). Perfect scenario would be an exact replacement, but I'll try anything!
Thanks...Rich