The FPA-4 represents the final FA design, combining the efforts of MLW (assembly), the Canadian General Electric Company Ltd. (electrical equipment), and the Dominion Engineering Works Ltd. of Lachine, Quebec (diesel prime mover). While MLW had produced FA-1/FB-1 and FA-2/FB-2 (and the passenger FPA-2/FPB-2) models for both the CN and CPR, only MLW produced the FPA-4/FPB-4 model. In the mid-1950's, as the march to 100% dieselization in the US was nearing completion, the Canadian railroads still had a way to go. Therefore, MLW's introduction of the Model 251 powered locomotive line found a more favorable market opportunity. In 1958, MLW introduced the passenger cab unit version of the RS-18, the FPA-4/FPB-4. At the time, the CN was in the midst of its final drive to dieselize its long distance and intercity passenger service, acquiring large numbers of locomotives from both GMD and MLW. The CN obtained 34 FPA-4's and 12 FPB-4's in 1958-59. Without a doubt, the most successful FA/FB's of all, these 90 mph passenger locomotives ran for 30 years on the high speed Montreal-Toronto corridor, from Montreal to the Maritimes and to the west from Toronto to Windsor and Sarnia. When VIA Rail Canada took over intercity passenger service, the FPA-4/FPB-4 operations expanded to CPR lines in Ontario to Sudbury and across Maine to St, John, NB. Their high speed prowess, though, could best be seen in Montreal-Toronto service. The CNR's first purchase of MLW passenger diesels consisted of six FPA-2's and six FPB-2's in 1955. In 1959, two FPA-2's and two FPB-2's were upgraded with Model 251B diesel engines, and renumbered immediately below the FPA-4/FPB-4 number blocks of 6760/6860 and above. The only external difference between the upgraded FPA-2/FPB-2's and the FPA-4/FPB-4's was the difference in the air brake system intercooler position. The FPA-2/FPB-2's intercooler heat exchanger was positioned on the unit's rear, as on all FA-2/FB-2's. The intercoolers on the FPA-4/FPB-4 were on each side immediately below the radiator housing, as indicated by the short Farr-Aire grill panel, and a spotting feature.

This design began with the construction of FA-1-FB-1-FA-1 demonstratoqs #’s 1500-1501-1502, completed at Alco’s Schenectady plant January 4, 1946. As originally designed, the grilled upper headlight was located below the upper seam around the nose, producong a continuous seam. The demonstrators were sold to the Gulf Mobile & Ohio in May 1946, the A-units becoming #'s 700-702, and the B-unit becoming B-1. A fourth FA-1 was built at the same time, retained by Alco and put>under static testing, but a labor strike at Alco interrupted this testing before any underlying problems with the locomotive design were identified. The headlight was moved to it’s now standard position for production DA-1’s. The first 20 production units were delivered with cast iron crankshafts in their Model 244 engines, since Alco did not receive its first samples of forgings until after the strike in mid-1946. The"forged crankshaft was rushed into production with limited laboratory testing and was incorporated in subsequent deliveries, with the forgings retrofitted into the GM&O units. There was no room in the FA-1 carbody for a steam generator, requiring that any railroad wishing to use the FA-1 in passenger service would need to team it with a steam generator-equipped FB-1. This lead to the 1950 era FA-2/FB-2, which addressed the FA-1 steam generator issue, introduced the 45 degree angled numberboard, and attempted to correct design flaws in the Model 244 diesel engine, by now revisions in design made it a Model 244D. By 1950, Montreal Locomotive Works ( MLW ) was an Alco lmcensee, and began production of the FA-2/FB-2 for Canadian customers. The Model 244 diesel developed some rather serious problems, mostly with the crankshaft and related bearings. Alco continued to refine the"224 design, culminating in the Model 244G. In 1949, Alco began design of a replacement, resulting in the Model 251 ( the 2 indicates the month of February, and the 51 indicates the year 1951, the date the firsr block was fired up in the laboratory ). This time Alco chose a more thorough testing procedure, thus the 251 was not ready for production until 1953. The first 10 Model 251 prime movers were placed in 78-ton export locomotives for Cuba, and Alco engineers shuttled back and forth between Schenectady and Havana fixing problems, mostly with cam rollers and turbochargers. By the time the Model 251 was ready for domestic production, the carbody style locomotive market had disappeared in the US, but the CN was still interested in this style, so MLW installed the 251 in the FA-2 style carbody and in 1958-59 the CN obtained a total of 34 FPA-4’s ( #'s 6760-6793 ) and 12 FPB-4’s ( #'s 6860-6871 ). When VIA took over the Canadian=passenger service April 1, 1978, these locomotives retained their CN numbers under VIA ownership, retiring in 1987-88.