It's a tough one, all right. Somewhere, there must be evidence of a press release about a particular train using "General Motors' new 2250-horsepower passenger diesel-electric locomotive." But where?
I thought somebody might have a book or magazine (Railway Age?) which contained this information. As far as I can tell, Trains never published anything like that, although there was a news item about the Canadian Pacific E8's replacing steam on the Montreal-Boston trains operated with B&M. (B&M had to wait until early in 1950 for its E8.)
The month of construction might mot be the month of first service due to time consumed in delivery. Lots of papers to sign before the unit goes out the gate at EMD. Also, Wabash, Burlington, and Rock Island could have taken delivery in Chicago on the same day the engine left LaGrange, but might have chosen to take delivery in another state due to tax treatment. And any of the railroads may have waited a day to put the new diesels on a particular train.
Perhaps there's a document archived somewhere by a historical society devoted to one of those roads, and - if so - it may be discovered while somebody is doing research.