Williams Big Boy

I have had issues like this on several Williams brass engines.  The most frequent issue is interference between the moving components and the fixed components on the body of the engine.  Usually a little filing will correct the issue and a small amount of flat black paint covers up the filed surface.

Roland

Big Boys, Challengers, etc. must have level track.  See attached photo, take a 24" level and check your track.  If low, use hobby sticks that look like giant tongue depressors.  They are available at Michael's or Hobby Lobby.

I had an issue with my Williams brass Challenger that may be worth checking out; RLAHIE touched on it - or the idea - above.

The underside of the smokebox and the forward steam pipes would interfere with each other in curves - the pipes would catch on a recess under the smokebox. It would come out of the curve looking like it had just taken a left hook to the face - it seldom derailed after this, but it was wanting to.

Assuming a similar construction technique (same brass builder), you might want to fill in the flat hollow under the smokebox with some brass sheet. I did this, and just used a good cement (Goo) to hold it in. It's invisible in normal usage - anyway, just paint it black.

Some simple filing may do it, as suggested.