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.
Usually, it seems like front trucks on most trains derail due to a lack of downward pressure, so if you bend the truck down just a bit, the weight of the locomotive pushes the wheels down onto the track.
Do that on a 999, however, and you’ll probably make the problem worse. At least it did when I tried that.
Flip a 999 over, and you see the front truck swivels on two shanks, rather than one like a certain other brand’s 2-4-2 locomotives. The rivets are really loose and the whole assembly has a lot of slop in it, to help it deal with less-than-perfect track. Through lots of trial and error, I found that these shanks need to be as straight as possible.
To straighten them without taking stuff apart, wrap the jaws of some pliers with some vinyl electrical tape and squeeze the shank between the pliers anywhere they seem to deviate from the straight and narrow. Expect to have to use several different sets of pliers. I find a needle-nose and a pair of locking pliers to be the most useful. The locking pliers take care of longer bends while the needle-nose work well for more precise work.
Expect it to take some trial and error. You’ll know when the shanks are straight enough when the train can go around curves without derailments. A healthy 999 can go around O34 curves for 30 minutes at a time or more without derailing, so if yours can’t, it’s time to reach for the pliers.