Traction motor sounds are what I have been working on getting, but considering I work on mostly DC powered units in yard service (save for when I start hostling soon), I don't get many opportunities to record them at work. If I was working the road I'd be able to get them. I have good GEVO-12 sounds that are ready, as well as FDL recorded by Matt Herman from ESU. But the thing holding me back is the traction sounds, as that is really what makes GE's sound neat. The DC GE's have a whiny sound when the motors are being powered (high pitched) which thankfully I do have the sound for. The AC GE's have a noise that the inverters make almost continuously like wind being blown through a tube, almost. It is really hard to describe without having a recording to back it up. 
Dash 9s and Dash 8s have the sound of the Contactors engaging when going into power, and a Pneumatic Circuit breaker sound (Psss-clunk) that occurs when you go out of power. ES44DCs do this too, but it is much quieter because the traction systems and high voltage cabinet on DC EVOs is in the Inverter Cabinet.
For EMDs, specifically the 70MACs which I would like to work with Wally on since he has much better recordings than I on, they have a unique traction system. They use the Siemens designed system, which when you are standing along the long hood near these two vents on either side:
mac.jpg
(Inverter Cabinets are here for each inverter)
they make a sound similar to German locomotives before they start moving, not the musical AC tones, but the blower sounds. Hopefully I can sneak onto a MAC soon and get it, as it is a rather neat sound.
 
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