The McIntosh & Seymour 531 was a diesel prime mover built by McIntosh & Seymour for use in railroad locomotives built by its parent company, the American Locomotive Company (Alco).
The 531 engine was designed and introduced in 1931.[1] It was a six cylinder engine, with a bore of 12.5inches and a stroke of 13inches.[1] It was produced in naturally aspirated and turbocharged variants, making 600 and 900 horsepower, respectively.[1] It was primarily used in Alco's line of HH series switcher locomotives, as well as the Rebel passenger trains.[2] It was replaced by the 538 engine, which had a similar output as the 531 but with upgraded components.[2]