Maine Central Railroad Class S locomotives were intended for heavy freight service.

They were the largest and most modern steam freight locomotives built for Maine Central; although former Boston and Maine Railroad 2-10-2s were later purchased to handle World War II freight traffic.

[2] All were built in American Locomotive Company's plant at Schenectady, New York and were numbered from 601 to 632 as delivered.

The United States Railroad Administration specified the USRA Light Mikado design for builders numbers 60933 through 60938 delivered in 1919.

The final six locomotives (builders numbers 65548–65553) delivered in 1924 returned to the original design with the addition of a booster engine which raised locomotive weight to 298,000 lb (135.2 tonnes).