Here are some further details, if anyone might be able to make a suggestion:
I noticed that if I had the trains going at a slow speed, they could go 4-5 laps around the oval before the roller problem would occur and I would need to give the roller a light press so it could spring back into position. Bumping up the voltage, the issue would happen before one complete lap.
I then discovered the problem is occurring at one specific spot--right where it exits the paper mache tunnel at one end (of course this is the most inaccessible area in the corner behind the Xmas tree). I squeezed back there and spent some time "erasing" that area of track as best I could with the track eraser that's intended for this purpose. The train will make the loop a few times, but inevitably the caboose will kick up a large spark when passing over this area and the roller will get fused against the rivet/housing above it. The top of the track is perfectly clean and oxidation-free, however. Weird!
Might I have to unscrew all the track sections in this area from the homasote bedding and clean the prongs that connect the tracks? Could oxidation on the prong connectors cause sparking when the train passes over?
