This photo is an extreme close-up of the problem. This was the mechanical problem I experienced immediately after installing the decoder. After about 15 minutes of working on it, I got it fixed and the engine ran nicely for about 5 minutes. Then it completely locked up again, with the same problem. The area where I placed the red circle in the photo is where the problem is. The L-shaped bar is supposed to be vertical (in this picture). For some reason it keeps popping out, which puts tremendous side-ways pressure on the wheel and causes the locomotive to derail and stop running.



Update: I finally fixed this mechanical problem. I repaired it one more time only this time I paid close attention to where the plug in the wheel needs to go. I subsequently ran the locomotive for an hour or two with absolutely no problems. As a matter of fact, I think the engine seems to run better the longer I run it. I don't know if that is just perception, or if it is a side-effect of the motor and gears getting worn in.

Two of the three sets of wheels on the rear truck of the tender are not in gauge. The rest of the wheels are.