It's even simpler than that, I banned low-profile MT flanges. Most metal wheels seem to work OK, the intermediate MT flange works, the pizza-cutters work. When you can try different wheels under the same cars in the same position in the same train....it's pretty conclusive. Isolate the variable in the equation to "Y", as in "Y did it derail?"
I'm mixing body-mounts and truck-mounts on the 89' flats surprisingly well, but the body-mounts have to stay toward the back of the train. They do stringline much easier than truck-mounts on the front.
