Glad to hear that your 8601 is running now.

The gear on the armature shaft is part of, and cut into, the armature shaft itself.

The gear and motor setup on that locomotive, and most diesels with vertically mounted motors, will not turn by hand when turning the wheels when fully assembled. Steam locos are manufactured with a different motor/gear setup, so you can turn the wheels by hand.

Use hardware store naphtha to clean off all old grease and oil from the armature, brush holder, gears, wheels, pickup rollers and track.
Replace any worn motor brushes if necessary. Oil both ends of the armature shaft, axle and gear bearings. Add a little grease to the gears, and the loco should run like new.

Larry