Athearn are a reliable brand. It sounds like your points may be too small a radius.
I would try to open it up and see if you can re-gauge the wheels you speak of.
Ideally an NMRA gauge is your best friend, but I've found turning the loco upside down and sliding a piece of set track, or identical kind of point over the wheels will show you what is going on.
No fun having a new loco fail on the layout.
Hi Mate, thanks for the feedback. This is an SD751. I'll take a good look at the bogies/wheel axles when I have some time. It seems like one of the middle wheel axles doesn't have any lateral movement. It could be a lemon loco, or just a design fault. Having been used to European locos (Trix, Brawa, Roco etc.) and Bachmann Branchline UK stock which are mostly great runners, I was expecting something better out of the box. Maybe 3 axle bogies aren't really suited to tighter radius curves.
Used to be everything would run on the USA standard of 18' Radius. Most newer USA models are designed to follow prototype radii. Athearn says the new locos can do 18" but advise 22". Is that an SD60? I would also check the wheel gauges. With my N scale locos I had a problem that with a wheel gauge tool the wheels seemed correct. When running these same locos through a switch it turned out the gauge was slightly narrow and the locos would pick at the facing points. American models are now using body mounted couplers as well, the body mounts make a huge difference in radius they can do. Also, the coupling are closer together and that also changes things a bit.
I tested my new Athearn CN loco both on my outdoor layout and my indoor test track. It seems that it really doesn't like curves or points. It constantly comes of the rails. I had to use 2nd radius curves on some sections, but none of my other locos have issues. Has anyone had any experience with this make of models before? It's a real shame because it's a stunning looking loco.