I just got a steam loco and it hands up on my track. My others work find. Is it had it needs larger turns and if what is the case what is the smallest that will work?
What type of Stem Loco? 2-8-0, 2-10-2, 4-8-2? What scale? These are important details if you want answers...
I'm guessing it runs fine on straight track? If it goes into a curve at speed, does it derail? If the answers to these questions are yes, you have it. Your curves are too tight for it. Is it a new engine? Or could some part not be swiveling the way it should?