Now, I haven’t checked this out on what is the matter here but I have this U25B by Stewart Hobbies (an older release) that I’ve had for some time but it had never been out of the box until just the other day. I’ve had many of my locomotives on my small switching layout just to see how they will run and how they performed on my tight radius turn track. I’ve had many six axle locos on it, including PA-1-PB-1’s, SD-7’s, SD-9’s and SD-45’s; they performed well. The U25B (a four axle unit) derails over some of the turns as well as some of the turnouts. I use code 100 Atlas track and turnouts. As I am in the throes of several structure building peojects, I haven’t had time to deal with the problem but before I did, I thought I’d get a little insight on it. My question is: is there a problem with the wheel sets, the truucks or my track , all three or some combination of these?
The first thing I would do is check the gauge of the wheels, second thing would be to set the engine on a piece of glass to see if “all” wheels are making proper and even contact with the surface.
My guess would be that either you have a wheel out of gauge or one them or the entire truck may not be sitting level.
If I remember correctly, these older Stewart/ kato drives have a protrusion on the truck cover. It is possible that you may be bottoming out @ turnouts and on magnets. If this is the case you can file the cover for clearance. Do this only after checking wheel guage, unobstructive truck swing and any other obvious trouble. Some of the Stewarts may have very stiff wiring to the truck power, these units don’t have the weight of say their Fs. It could also be a possibility. Wheel guage or truck swing/ movement is most likely the trouble to check first.