I had a hunch this was where you would find the problem. The bushing cannot corrode, but the axle within can do so. It is either that, the lubricant which has age failed, or perhaps a small accumulation of foreign matter from years of use. Repair parts are available, if all else fails.
Yay! I set it in the penetrating oil for an hour and it was still stuck tight. Using great caution and a light touch with needle-nosed pliers, I was able to slightly rotate the plastic piece. I worked this same dunk/wait/slightly rotate cycle a half-dozen times and the plastic piece began to free up and after three hours or so, all was buttery smooth. I put a few drops of LaBelle plastic-safe oil on the metal shaft and the gears, reassembled the truck and reattached the motor after putting a small daub of grease on the motor pinion and it's mating gear in the truck. All is well and it runs with renewed pride on my mighty yard-long test track. I've cycled it back and forth only a few hundred times.
I picked up my copy of Greenberg's Repair and Operating Manual for Lionel Trains 1954-1969 (Sixth Edition, Second Printing c. 1990) from my local library and I am very impressed. In its 736 Pages are Lionel factory assembly drawings, original repair guides and parts diagrams for every Lionel train product of the post-war period, even track components, specialized Dealer tools, trucks, transformers and accessories. It's neat to see how the many accessories were made to work -- Lionel's design engineers were a creative and highly skilled bunch.
It looks great! I had a friend in grade school who had Lionel trains and he had a couple of those F units. Doug
