I have to agree with Ben's observation about this being a subject begging for a researcher.  I currently work for the Army as a civilian, have for over 23 years, with four years of active duty before that.  Trying to track when markings switched to DODX, and on what equipment is for a braver soul than I. 
About nine or ten years ago I had the fun task of identifying what rail equipment the Army had (freight cars and locomotives) and where it was.  At that time much of the rolling stock dated back to WWI - outside braced wooden boxcars.  Locomotives included a variety of
EMD and Baldwin switchers, some other smaller industrial units, and a handful of GP-9 and GP-40's bought secondhand.
With the exception of the GP units, everything (rolling stock) was pretty much confined to in-plant switching at the ammo production facilities, while the locomotives were scattered at installations across the U.S. 
Steve Thornton




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