I have a few old Atlas SD-35s that were in need of repair and I'd hoped to do it inexpensively, but discovered something about Atlas motors that has added to the complexity. I knew that Atlas had the original Fast Speed motors (identified by the typical black plastic case around the motor) and a Slow Speed motor introduced perhaps 10 to 15 years ago (identified by gray plastic case around the motor). What I didn't know was that Atlas has introduced an even slower Scale Speed motor which runs super slow. Unfortunately it has the same black plastic look as the Fast Speed motor. So, Atlas has three types of motors, each with a different speed. I've found a visual cue that might help distinguish between the old Fast Speed motor and the newest Scale Speed motor. As noted, both have a similar black plastic case surrounding the motors, but parts within the Fast Speed motor's rotating assembly have a graphite-colored look and the Scale Speed motor parts within the rotating assembly have a greenish tint. At least this is the case with the motors sitting before me. Perhaps this will help you to identify what you need if you are seeking to speed match another locomotive in your collection. This is of major importance to us DC operators and the speed differences between the three types are significant enough to perhaps be important to DCC owners too.