Place the engine on a clean shiny countertop and backlight it well with a strong lamp. Get down with your eyes to where the flanges meet the counter surface and look for light where there shouldn't be between any between the wheel flanges and the counter. If may not be a twisted truck frame, but a bad axle, flashing on a bolster between the engine frame and the truck, a wire snagging a component of the drive or truck...you'll have to eliminate the obvious, but it usually means partial disassembly.
Also, whenever I have had such problems, also with diesels, and also with Genesis, I find that the outer rail on my curves is as much to blame. I found that, whereas all my other period diesels and steamers of various makes and models were happy with my tracks, the Genesis SD75M's were not. So, I wet the ballast and waited for it to soften (10 minutes) and then gently pried up the outer rails along the place where the engine derailed. I placed the odd sliver of plastic or wood, or cardstock under several ties, tamped down the ballast to force some of it under the ties that didn't have new supports, and then simply waited for the ballast to harden once more. I am talking about raising the rail tips on the outer curve by maybe 1/32" or a bit more.
That seemed to make a difference. I no longer get derailments, and my steamers and period diesels from the transition era don't care either way.
So, I would also have a very determined and considered/objective/critical look at my track levels. Find a worthy straightedge and place it atop the rails just before and over the place where your new diesel derails. Backlight the straightedge and get your eyes down to rail-top level. Do you see light where you had thought there'd be none? There-in lies your troubles.
-Crandell