GunnerV's description of gear ratios is most factual. I might add the following; the ratio (62:15, 65:12, etc. is always constant because of the center distance between the driven axle and the traction motor is constant. EMD went from 40-inch to 42-inch wheels but it didn't change the gear ratio...it allowed a greater distance between the rail/ crossing surfaces to the bottom of the motor frames as well as extended wheel life (40-inch diameter x 3.1416 =125.66 inch circumfertence vs 42-inch=131.94 inch circumference.) Less wheel revolutions between Los Angeles and Portland means less wear=savings.

Older traction motors had a certain short time amperage rating. Back is the 50's when the SW-1's were being built they didn't need traction motor blowers...the D15 main generator couldn't produce enough current at 600 hp to overheat them. The SW-8's at 800 hp did have blowers...however once the D27 and 37 motors were replaced by upgraded motors (most skipped the intermediate models and used D-77's) those blowers were deactivated because an individual motor could handle 500 hp or more.

An E-unit was basically two SW-9's in a covered wagon carbody. Due to the highrt gear ratio, they didn't have to produce a lot of tractive effort but were designed to get a fairly light train up to speed. The A1A-A1A configuratiion was mainly to distribute the weight of the heavier carbody over more axles to get the weight of the unit down to acceptable axle loadings of the time. If they were designed for tractive effort they would have had the center axles powered making them a C-C six motor unit. The problem with this configuration is (1) with the motors in series they would have had a lower speed/transition curve and (2) in parallel would have had a shorter "short time rating". Over the years these "shortcomings" would be overcome with the introduction of the SD-series.

So far as units with the optional 65:12 ratio; they were only good for switchers or heavy transfer locomotives as their top speed was somewhere around 45 mph. The biggest objection to the 65:12 ratio was that the traction motors required that the 12-tooth pinion had to be machined as part of the armature shaft which was extremely costly as the whole motor armature had to be replaced in the event of a pinion failure (which was quite common due to the amount of wear they experienced due to their higher rpm).

Thought this might clarify questions that might still linger.

Shortline Sammie