The FA-1/FB-1, which featured a 1,500 horsepower (1,100 kW) rating, was built from January 1946 to October 1950 with a 1,600 hp (1,200 kW) version produced between March and August 1950 (many early models were subsequently upgraded to 1,600 hp). The 1,600 hp (1,200 kW) FA-2/FB-2 (along with the FPA-2/FPB-2 variants) was built between October 1950 and June 1956. The 1,800 hp (1,300 kW) FPA-4/FPB-4, powered by the 251V12 engine, was built between October 1958 and May 1959 by ALCO's Canadian subsidiary, Montreal Locomotive Works (MLW). Externally, the FA-1/FB-1 could be distinguished from the FA-2/FB-2 (FPA-1/FPB-2) by the position of the radiator shutters — the FA-1/FB-1's shutters were at the far end of the carbody, whereas on the FA-2/FA-2 they were further forward, the design having been modified to allow the installation of a steam generator behind the radiator. The New Haven owned 30 A-units and 15 B-units, and were first used to replace the 2-10-4's on the Maybrook. They soon found themselves in both passenger and freight service throughout the system. Reskin by Jesse DuBois. Original model by John Maton.