derailment

[ac12]

I did not see where you described exactly where the derailments occur, what kind of track is there (turnouts, crossings, 19"R curves, 9-3/4"R curves, etc.  Put something at the track wherever it derail, and after a while you might see a pattern.  This helps to troubleshoot the problem.

Is it the pilot truck/wheels or the first driver wheels that is derailing?

As was said by other, check that the wheels are in-guage then check your track.
I had some cars that would derail going thru a turnout.  Turns out the wheels were picking the frog and going the wrong way.  Fixed the guardrails and the problem went away.

The center of gravity (CG)/balance point of many steam engines is rear of center.  This causes them to want to pull a "wheelie," which is asking for a derailment.  I stuffed lead or tungsten putty into the smokebox to move the CG forward.