The ALCO S-1 and S-3 were 660 hp switcher

Switcher

A switcher or shunter is a small railroad locomotive intended not for moving trains over long distances but rather for assembling trains ready for a road locomotive to take over, disassembling a train that has been...

 diesel-electric locomotives produced by ALCO

American Locomotive Company

The American Locomotive Company, often shortened to ALCO or Alco , was a builder of railroad locomotives in the United States.-Early history:...

 and their Canadian subsidiary Montreal Locomotive Works

Montreal Locomotive Works

Montreal Locomotive Works was a Canadian railway locomotive manufacturer which existed under several names from 1883–1985, producing both steam and diesel locomotives. For a number of years it was a subsidiary of the American Locomotive Company...

 (MLW). Basically, the two locomotives differed only in trucks

Bogie

A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...

, 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 540 completed, while the S-3 was constructed between February 1950 and November 1953 (MLW until 1957) with total sales of 292.