In order to properly function on model train narrow radii, the centre axle on the Maerklin Ae 6/6 (3050/3332/30501) is designed so that the wheels on that axle have different flanges that allow the bogie to rotate on the tight radii without forcing the train off the tracks.

I do not have the 30501, so I am basing my comments on my 3050. I am not familiar with springs on the wheels. There may be contacts to connect the axles to the body of the bogie insert, which then connects to the chassis, providing the return voltage (brown) contact between rail and motor.

If the axle is not freely rotating, it could be that something is stuck interfering in the motion of the axle, the axle could be bent (should be visible when rotating) or the slot in the bogie insert that the axle passes through may be compressing against the axle, impeding motion.

The part number to replace the bogie insert with wheels is E216980

If your model is within warranty, you may want to contact your dealer or service(at)maerklin.de

Regards

Mike C

Just like Mike C suggested, the middle axle should be running *very* freely.
If it's not the case, something must be caught between the wheels/axles and the chassis of the bogie.
It's easy to check!
Unscrew the pick up (long screw!) , and lift the contact plate.
Be careful not to break the contact wire!!
Please notice that this piece of plastic has notches corresponding to those on the bogie.
Any obstruction/ debris/ pich/ bend/ etc...should be easy to spot/diagnose now.
Reassemble in the reverse order.
Voilà.
Cheers
Jacques

I took it apart as Jaques suggested and it looks fine, no binding. The wheel appears to be free and spins freely by hand.
I checked the plastic part of the bogie, and even bent it slighly to make sure it isnt rubbing.
Result: no change.
So for some reason the wheel intermittently stops turning.
Ill monitor it, continue to look for a reason.

Thanks for the suggestions,

Don
.

Yes - I have had the same thing as well.