This design began with the construction of FA-1-FB-1-FA-1 demonstrators #’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, producing 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 FA-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 licensee, 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 first 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.

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.

The LRC (Light Rapid Comfortable or Léger Rapide et Confortable) was a product of the Rail and Diesel Products Division of Le Entreprises de J. Armand Bombardier Ltee (Bombardier or BBD), which had purchased Montreal Locomotive Works (MLW) in 1975, which in turn had been an Alco diesel locomotive licensee since 1948. The project began in 1972 as a joint venture between MLW, Alcan Canada Products, and Dominion Foundries and Steel Co. (DOFASCO). In 1973 MLW constructed a prototype locomotive and coach. The locomotive has a 12-cylinder Model 251F diesel which produced 2900 horsepower, 2000 hp for traction, 900 for auxiliary power (Head End Power, or HEP, for coach heat/air and lighting). The prototype tested for over a year at both the CNR Research Center and the US DOT Test Facility in Pueblo, CO. Although the tests were favorable, orders were not immediately forthcoming. In February 1977 Amtrak entered into a two-year lease-purchase agreement for two trainsets from the Canadian Government, which ordered them from Bombardier. The trainsets consisted of one locomotive and five cars, in accordance with MLW’s recommendation that a single locomotive was sufficient for five cars or less. As a result of the prototype testing, Bombardier made a number of significant design changes, one of which was the use of a 16-cylinder Model 251F diesel rated at 3725 hp, 2700 of which was available for traction. Amtrak’s order was scheduled for delivery in early 1979, but a seven month labor strike at Bombardier caused it to slip into 1980. Also in 1977, VIA Rail Canada, still in its formation stage, issued a solicitation for new high-speed trainsets. Bombardier was selected and an order for 10 trainsets (20 locomotives, 50 cars) was placed for delivery in 1980, but because of the strike, actual delivery began in mid-1981. A series of mechanical problems plagued initial service on the Montreal-Toronto, Montreal-Ottawa, and Toronto-Sarnia service delaying full-scale service into early 1982. VIA classed its trainsets as LRC-2 and numbered the locomotives in the 6900 series. Deliveries on this order were completed in 1982, and the following year VIA placed an order for an additional 10 locomotives and 50 cars, classed LRC-3. The second order was delivered in 1984, and locomotive #6930 had the distinction of being the final locomotive built by Bombardier for domestic service. All have since been retired.

The objectives of the LRC (Light Rapid Comfortable) design were to produce a high-speed, lightweight, low center-of-gravity passenger train that overcame the deficiencies of the sucession of previous designs. Thus, the features of the LRC design were: - Power-banked coaches of 85 foot length, electro-hydraulically tilted up to 10 degrees on curves - Coaches 12 feet high with an underbody clearance of 8 inches, similar in dimensions to the TurboTrain, but with an air bag suspension and heavy acoustic insulation - Coaches constructed primarily of aluminum with a stressed skin exterior for light weight. Their 85 foot length provided greater spaciousness and minimized the claustrophobic feel of their predecessors - Individually-coupled cars, each with a vestibule on one end, providing flexibility for train consists, in both single-ended and double-ended operation - Matching low center-of-gravity locomotives - Train speeds of up to 120 mph, enabling 100 mph terminal-to-terminal speeds on existing mainlines with 4 degree curves. The locomotives were powered by a 16 cylinder 251F prime mover producing 3725 hp total, 2700 available for traction, as the locomotive supplied all of the power for train heating, lighting, and other utilities. This arrangement avoided the use of a separate power unit as existed on many other installations, and thus generated less noise than was typical with separate HEP power plants. But by 1988, VIA's operational assessment of the LRC was more favorable to the cars than the locomotives. Their complex electrical system was particularly cited as failure prone. At that point, VIA's expectation was only a 15-year life for its fleet, and indeed, by 1995 about 20 of the 30 locomotives were in storage. The LRC's only slightly outlived VIA's prediction, the last 7 operational locomotives were retired at the end of 2001.