Maine Central Railroad Class O locomotives were originally intended for heavy freight service.
They proved so well-suited for branch line service the design was among the last steam locomotives built for the Maine Central.
The first were built in 1903 at the American Locomotive Company (ALCO) plant at Schenectady, New York with builders numbers 27657-27659 and 29029–29030.
[2] Builders numbers 40576-40579 and 41235-41239 were assembled with Walschaerts valve gear in ALCO's Rhode Island plant in 1906.
[2] When the Maine Central began purchasing diesel locomotives, switchers intended for branch line use were numbered in the 300 series reserved for the O class.