Derek,
if if you turn the loco around, does the same truck (which would now be on the front) derail, or does the following truck still derail? If it stays with the same truck, probably a wheel is slightly out of gauge.
On my engine, the problem turned out to be the pickup roller, & if I looked closely, I could see the roller drop & snag on the edge of the rail as it passed through the switch, but I might have missed it had I only been looking at the wheels.
Good luck,
Bill in FtL
