Yes, sometimes brushes are the problem, but probably not in this case. And yes, parts are readily available, and people do convert their locomotives to can motors and solid state reverse units, but some of us stick with the good old '50s technology.
The problem is in the tender, where the mechanical reverse unit controls power to the motor field and armature. The contact fingers are either corroded, or worn, and can pretty easily replaced. The wheels don't turn when you push on them because of the worm drive mechanism--when new there was a tag on them that told the new owner to not try to turn the wheels.
There's lots of online information available--I am surprised you did not find any of it! The Yahoo group is S-Trains, and one great source for information and parts is : [www.portlines.com]
If I can help any, feel free to contact me; djdewey@cncnet.com I've been fixing AF for some 45 years--although those first few years were pretty haphazard--I was only about 10!
S'incerely,
David Dewey