The ALCO S2 and S4 were 1,000 horsepower yard switcher diesel-electric locomotives produced by ALCO and Canadian licensee Montreal Locomotive Works (MLW). The two locomotives differed only in trucks, with the S-2 using ALCO's own Blunt trucks, and the S-4 riding on standard AAR type A switcher trucks. Both units were powered by ALCO 539 turbocharged, 6-cylinder diesel engines. The S-2 was built between August 1940 and June 1950, with a total of 1502 completed, while the S-4 was constructed between June 1949 and August 1957 (MLW until 1957) with total sales of 797. Canadian production of the S-4 started more than a year before U S production of the S-4. ALCO did not start building the S-4 until August 1950. A modified version, the S-7, was built by MLW only; 29 examples of the S-7 were built between June and August 1957.