Interesting that Mike Regan mentions the plastic gears & brackets; guess Lionel knows they have a materials durability problem.
I decided to tear into the transformer. After all, what could go wrong??
After removing the pilot light assembly (glued in place with hot melt glue) I unscrewed the two self-tapping screws you mentioned and the Pot assembly was easily lifted out for inspection. The tricky part to master was to align the teeth of each pot with the voltage markings corresponding to the A-U and B-U handles.
It's getting late here so I'll tackle the epoxy tomorrow. That's a one-way dead end. If that fails, removal requires breaking irreplaceable plastic parts....Hmmm...I think I'll sleep on this one.