The ALCO RSD-4 was a 1,600 horsepower (1.2 MW) six axle diesel-electric locomotive built by the American Locomotive Company between 1951 and 1952. It was a derivative of the four-axle ALCO RS-3, with two additional powered axles which allowed better tractive effort at lower speeds. Due to the inadequate capacity of the main generator, this model was later superseded in production by the ALCO RSD-5.