Years ago, I brought a Lionel 2026 locomotive in for repair that had belonged to my dad. It ran poorly, and either dad or his kid brother had taken it apart at some point and lost some of the parts, including the front truck.
And then, when I got the locomotive back and I put it on the track for a test run, it derailed constantly. The front truck just wouldn’t stay on the track, no matter what I did.
When I took the train back to the shop, the co-owners argued a bit about what might be going on. One guy said something must be wrong with my track, but the problem with that theory was that I had other locomotives, like the nearly-identical 2037, a lowly 1110, and a couple of Marx locomotives, that ran just fine on my track.
The other guy pulled out a caliper and measured the distance from wheel flange to wheel flange–the thicker, inner part of the wheel that sits between the track. It should be very close to 1.25 inches. Mine was nearly 1/8 of an inch too narrow, so the fit was too sloppy and the wheels could slip off the track.
The problem is a bit more obvious if the distance between the flanges is too wide. That won’t work right either, but you’ll be able to see and feel that the wheels didn’t fit right.
Adjusting the gauge is usually just a matter of bending the truck frame a bit. There’s a fair bit of play in it. And chances are, that minor adjustment is all your locomotive will need to work reliably. That was true for mine.