The motors will fail if the brush holder gets overheated. if you've seen smoke at higher speeds and the switcher still runs, this is probably what's happening; the motor is no good. this 3-pole open-frame motor (somebody keeps referring to these motors as 'pancake motor'- not true- a pancake motor is one that has a flat, or disc-shaped commutator) is barely sufficient to power this switcher. it's used to keep this product within a particular price point. there has to be a better way without going to a can-type motor. what i did when one of my MDTs smoked itself, was get a surplus spectrum wide-cab (too new for my 60s layout) and use the motor and one of the sector worms from it. remove the flywheels from the 5-pole motor and remove the brass worm from one sector shaft. install the worm onto the new motor nose shaft and test the switcher with the body off. do not cut the rear shaft until you are happy with the worm placement and how it runs (sans body), since that rear shaft gives you a 'handle' for working with the motor until you are finished testing. when you're done, you can trim off that extra shaft and you'll like how your little switcher performs. i think mine runs as well as the can-motor MDTs the guys run at my club. If you are unfamiliar with working on tiny motors and shafts or don't have the tools and fixtures for such tiny work, get someone who does give you some help. Once you've seen it done, you'll want to dive in and do for yourself. If you aren't careful. you might have fun while learning!! good luck!