“This lead unit of the four-unit EMD-103 demonstrator locomotive became the prototype of the first mass-produced diesel-electric locomotives used for freight service in the United States. They rapidly replaced the steam locomotive. Called ‘the train that did it’ in a February 1960 edition of Trains magazine, it was a revolutionary step for the rail industry.”
This 4-unit set was tested on various railroads, including the Santa Fe in southern California, beginning in 1939.
For a full history, see http://trn.trains.com/railroads/railroad-maps/2016/08/1939s-big-breakthrough-the-ft .
Diesel-electric (diesel engine spinning a generator that powered electric motors on the axles) locomotives were successfully used in U.S. passenger rail service earlier in the 1930s.