Hi,
After just cleaning, and lubing with pfte and reversing the shim it startend sqeeking again.
So:
I sanded carefully the axle end with 2500 grit sandpaper.
Then I used polish paste. Both some leftover from car bumper repair.
I used a micosope on 50x to inpect it after each polish rond.
The amount of material taken of from the axle is minimal.
So far no sqeeking.
Pic below from the axle end.
I kinda wonder how this damage happend. Its copper contacts were in better shape then my childhoods train motor. I dont think it has ran that much (I was quite a fan as a kid - still are) Also the axle end and shim that was damaged was the forward direction.