The tires are being taxed beyond their ability to grip the wheel and grip the rails - and
the grip of the rails is winning.
If your tires are older, they are stiffer and will come off. If your train is very heavy,
and the tires are tired (as it were), they will come off. If your tires are new but the
train is too heavy, the tires will come off. If the grades are steep and the train is
heavy....you get the picture. Curves can be as bad as grades.
First, try a little CA (Super Glue) under each tire; no luck? try a shorter train; no luck?
try new tires; no luck? you've gone beyond the equipment's design parameters. The track has nothing to do with it - but if you've gotten oil from a sloppy lube job under the tires, it can contribute.
The main thing that we must remember, though, is that: these are not real trains
and locomotives; they are models, and have their own limits. Don't abuse them.