Fix a Marx 999 that derails a lot
Marx’s most popular locomotive might be the 999, because it can pull anything Marx made–6-inch tin, 7-inch tin, 3/16-scale tin, 4-wheel plastic, and 8-wheel plastic–without looking out of place. It really only has one problem: The front trucks on many 999s are prone to derailments.
Counterintuitively, the fix for a 999 is the opposite of how you fix the same problem on many other O gauge electric trains.