In 1923, the M-1 Mountain was built as a classof heavy mixed-traffic locomotives of the 4-8-2 “Mountain” wheel arrangement. The wheels consisted of four pairs to drive the locomotive, a four-wheel driving truck in front for stability, and a two-wheel truck to support the large firebox needed for sustained power. Although built for both passenger and freight work, these locomotives are most remembered as high-speed freight trains.

To test the design of the train, a single prototype was built and tested for three years by the Pennsylvania Railroad’s Altoona Works. Reports were favorable, and 200 of the design were ordered by Baldwin Locomotive Works and Lima Locomotive Works.

M-1s remained in service on the PRR until 1957, when steam was phased out on the line. An M-1 is currently on display at the Railroad Museum of Pennsylvania, and was added to the National Register of Historic Places in 1979 as Locomotive No. 6755.