After giving it some thought, it almost sounds like you have a split or slipping worm gear. The damaged gear slips when you go in one direction but is ‘okay’ when traveling in the other because the stress on the gear changes when you reverse the loco’s direction. If you can, remove the engine’s plastic shell and hopefully you’ll be able to see the gear train as it runs. It should be fairly evident to see if the gear is turning smoothly or not. If the gear is damaged you may be able to get a replacement from the manufacturer but the actual repair could be difficult depending on your overall experience.

Hope this helps,

Geof