We have two of these locomotives. They both have the exact same problem. Both have plenty of front to back axial motion in their trucks to allow them to traverse minor deviation (bumps) in the track. Neither has any side to side axial motion. This causes them to derail on any section of the layout that has even the smallest side to side deviations (level mismatches, etc). It also causes them to 'pick' switches more often. None of our other locomotives (nearly 150) have these derailment problems, only these two. All the rest of our locomotives (all are HO) have plenty of front to back axial truck movement and plenty of side to side axial truck movement. We can send them back to Athearn, but they will jus replace them with the same locomotives, without fixing the problems. As the problem exists in two SD40's, we are guessing it is a 'model problem' affecting only the SD40's. Is there a fix? The locomotives, other than this problem, run great.
Thank you
Check the frame of the trucks on them.If you find that the frames are bent outward... (the wheels are not lining up properly in the holes of the frame)....take a needle nose pliers and CAREFULLY bend them inward..this will cause the wheels to slide properly in the holes of the truck frame.I had the same problem with my IC SD70.Hope this helps.
If that doesn't solve it, you might try posting your question somewhere that gets more visitors, like here.
Good Luck!