I just bought 2 Lok’s which where sold as defective.
One was a BR52 (3415), which was “running wobbly” and the other was a BR E91 (3628),
which where suposed to be electricly defective.
It turned out the BOTH lok’s have at some point had their wheels removed at put back on again. But unlees you know what you are doing this can lead to big problems, and with my 2 lok’s it certainly did.
Both lok’s had wheels so loose that you could ealisly remove them with your fingers.

The only true way to get them back on firmly is to “knurl” the axles. Any type of knurlig will work as long as it adds to the overall thickness of the axle. See how smooth the axle was on my BR52 – that would never grab on to the wheel. Of course you need to rough up the axles evenly all the way around, or the whell will not sit center on the axle and the lok will run wobbly again 🙁
If you are unlycky, the hole in the wheel has become so large from all slipping, that not even knurling will make it grab the wheel firmly – then you need to get a new wheel.

I did not have the possibility to knurl the axles, so I mailed them to a friend which did a good job. Here you see the result.
Now all I have to do is press the wheel back on. To that I will be using a small vise, and take care that I press evenly on the whole wheel or at least evenly on the rims.
Also remember, if your lok’s have tie rods, the the wheels must be set at an angle of 90 degrees from each other.
The distance between the inside of the wheels should be 13,8mm for Märklin AC.

What have I learned from this….
People who remove wheels and dont know what they are doing, are fools !!!
– And, dont buy lok’s with loose wheels unless you have the tools to fix it.