As I mentioned above, there is more than rotational balancing that must happen to spin the running gear at such a high RPM. The thrust of the pistons will make the locomotive hunt back and forth until it derails. This piston thrust force can be controlled by rigid centering devices on the lead truck of the engine (the N&W did this on its 4-8-4's), but that makes the locomotive much less flexible in negotiating curves. You could tighten down the pony truck on a 480 to make it track fairly straight at 40+mph, but it would not be able to take the 20 degree curves it was designed to navigate.