The ALCO RSD-15 is a diesel-electric locomotive built by the American Locomotive Company (ALCO) of Schenectady, New York between August 1956 and June 1960, during which time 75 locomotives were produced. The RSD-15 was powered by an ALCO 251 16-cylinder four-cycle V-type prime mover rated at 2,400 horsepower (1.79 MW); it superseded the almost identical ALCO 244-engined RSD-7, and was catalogued alongside the similar but smaller 1,800 hp (1.34 MW) RSD-12, powered by a 12-cylinder 251-model V-type diesel engine.

The ALCO 251 is a 4-stroke diesel engine that was developed by the American Locomotive Company to replace its 244 and 539 engines. The 251 was developed to be used in diesel locomotives, as a marine power plant in ships, and as a stationary power generator.