Maine Central Railroad Class W locomotives were intended for heavy freight service. They were of 2-8-0 wheel arrangement in the Whyte notation, or "1'D" in UIC classification. They replaced earlier class O 4-6-0 locomotives beginning in 1910. They were in turn replaced by class S 2-8-2 locomotives for the heaviest freight service beginning in 1914, but remained in use on lighter freight trains until replaced by diesel locomotives after World War II.