Gene you're lucky that the issue is cosmetic. The driving wheels on MTH steam locos are made of soft stuff. If a loco has a lot of run time on or below its minimum diameter, the flanges wear to a sharp edge, and it begins to derail at switches. I've heard this from a few different reputable sources. The worse news is that would take real skill to replace the front driving wheel(s), or you could buy a whole new "drive block" if one is available. Otherwise, it's a shelf queen!
Personally I find MTH locos to be gauged "tight." Mike Wolf wanted to sell trains, and he was pretty aggressive with the published minimum diameter. If you put a lot of miles on and want best performance and longevity, it would be wise to buy something rated for curves one size smaller than what you actually have. My $.02.
