A recurring issue on some MTH steamers with a 2-wheel front truck, apparently. I have had to "re-design" the front truck mount on the RK 2-8-8-2 a couple of times. The main mounting screw/pivot is in a slot, cast into the truck; this allows the truck to attempt to swing on both ends, "pivoting" around an imaginary center point, causing it to cock sideways and derail, especially on switches and the like. I epoxied a washer over this slot, giving the truck a just a plain hole for the screw.
My 3rd Rail NYC Mercury 4-6-2 had a similar issue with its rear truck. Same cure.