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The ALCO S-1 and S-3 were 660 horsepower (490 kW) switcher diesel-electric locomotives produced by ALCO and their Canadian subsidiary Montreal Locomotive Works (MLW). Basically, the two locomotives differed only in trucks, with the S-1 using ALCO's own Blunt trucks, and the S-3 riding on standard AAR type A switcher trucks. The S-1 was built between April 1940 and June 1950, with a total of 543 completed, while the S-3 was constructed between February 1950 and November 1953 (MLW until 1957) with total sales of 300. A modified version, the S-10, was built by MLW only; 13 were built between January and June 1958.
Many such locomotives, both private and goverment (Army, Navy, Air Force) were used in harbors, depots and shipyards from the World War 2 era up to the modern times, with a number of private railroads still operating the type. A few locomotives were used in other countries, except USA and Canada some were sold to Mexico, Brazil, and UK.