In 1940, General Electric and American Locomotive Co. (Alco) concluded a sales and marketing agreement to manufacture diesel locomotives under the Alco-GE label. While World War Two prevented immediate building, the Alco-GE planning efforts continued.

In the 1940s, EMD's success with their "FT" freight diesels drove ALCo to develop their own streamlined freight locomotives. These diesels were developed around ALCo's new turbo-charged "244" prime mover, GE electricals and AAR type B trucks. As a result, the first FA/FB set was delivered to the GM&O in 1946. The 1500-h.p. FA1s and FB1s sold quickly, and were succeeded in 1950 by the up-powered FA2s and FB2s.

Also in 1946, Alco delivered their first streamlined passenger diesels to the Santa Fe. These 2000 h.p. units in cab/booseter configuration were later designated the PA/PB-1 type. Upgrades in 1950 resulted in the PA/PB-2 type. Some units were later rebuilt and poularly called PA/PB-4s.

In 1950, Alco upgraded their specifications to DL-212 (cab) and DL-213 (booster). Since these units were designed for dual freight/passenger service. To accomodate steam generators, both units were lengthened, cabs to 53'6" and boosters to 52'8". these units were rated at 1500 h.p. The cab units were 51'6" long and the boosters were 50'6" long, leaving no roon for steam generator equipment. All DL-208/209 units went to the Gulf Mobile and Ohio. Units built to later specifications (DL-208A/209A, DL-208B/209B and DL-208C/209C) were more widely distributed. Units with steam generators installed were designated FPA-2 and FPB-2.

In 1951 through 1953, the Montreal Locomotive Works produced FA-2s and FB-2s under Alco specifications ME1600FA and ME1600FB. These were produced for Canadian National Railways and Canadian Pacific Railway in 1951-1953.

Alco FA-2s and FB-2s built with steam generators installed were designated FPA-2 and FPB-2. Only the Missouri Pacific and some Mexican railroads had FP models built at the Schenectady plant. All other FPs were built by the Montreal Locomotive Works produced FA-2s and FB-2s under Alco specifications ME1600FA and ME1600FB. These were produced for Canadian National Railways, Canadian Pacific Railway, And National de Mexico in 1953-1955.

Montreal locomotive works also produced the FPA-4 and FPB-4, under Alco specifications DL218 and DL219. Not only were the engines uprated to 1600 h.p., car bodies were stretched to 54'0" (cab) and 53'2" (booster). These locomotives were built exclusively for Canadian National Railway.