The 660-horsepower S-1 was the first of a line of four switchers that totaled about 30% of the American Locomotive Company's total diesel production. Powered by a six-cylinder McIntosh & Seymour 539 diesel engine, the S-1 could be found working on railroads from coast to coast. Production of the S-1 lasted from April, 1940 to June, 1950, with 540 units constructed for the US and Mexico. The units are easily distinguished from other Alco S-series switchers by their radiator shutters (taller than they are wide) and Blunt trucks. So useful were these switchers that the War Production Board allowed their continued production during W.W. II.
 
Versatile S-1 switchers could be found working yards and local runs for many roads and industrial owners. Regular haunts included steel mills, paper mills and other large industries, and some roads used them to haul logs. By the 1980s and 90s, the remaining units in service worked for industries and shortlines, with a few continuing operation into the 21st century.