Engine will go in reverse, but wheels just spin going forward. Removed the armature and the botton bushing where the tip of the armature worm gear shaft sits in is worn and damaged. Thus, when the engine tries to go forward, centrifugal force is pushing the armature shaft away from the truck's worm gear so the two are not meshing and the wheels just spin. This repair is beyond my capabilities and comfort level --- I wouldn't know how to access it or drill out the remnants of the old bushing. Who do you recommend for repairing this MPC-era engine, preferably in-house rather than a business that farms it out to others.
This is a follow up to my original post to let you know I took on the repair myself. Using a drill bit the approximate size of the bushing, I discovered that it is plastic and not a bronze bushing so it came out quite easily. The replacement part number from The Train Tender is "8855-162, armature bearing insert". Put Red n' Tacky grease on the worm gears and bearing insert and the engine now runs in both directions.
The worm gear on the wheel axle was cupped a bit from the slippage between it and the armature shaft. Using a file, I removed the burrs from the grooves as best as I could. I also noticed that the verticle play of the armature was probably more than it needed to be, and maybe why the original bearing insert got worn out. So I used a .050" allen wrench to turn the set screw down a half turn or so until the verticle movement was between 1/16" to 1/8".