Good Evening,
I am by no means an expert in this area and simply read all the things you have already tried. I would say try looking at the area in the motor where the threaded shaft goes into the drive wheels. I don't know how often you run this particular locomotive but if it's been put away or not run often this could be the culprit. If, in fact, this is the problem try using LGB grease or similar in that area. The grease gets old and may need a good old re-greasing if that's even a word. Best Wishes!
Dave
