The Mountain type of locomotive, with a 4-8-2 wheel arrangement, first appeared on the Chesapeake and Ohio Railroad in 1911 and excelled at hauling heavy freight trains at high speeds. The Pennsylvania Railroad adopted the M1 design in 1923 to replace its slower fleet of L1 and II locomotives, using technologies and designs proven in earlier, smaller models even though it was larger than anything the railroad had built before. Originally intended for both freight and passenger service, the entire M1 fleet carried the keystone-style number plate usually used for passenger locomotives, but the class eventually served exclusively in freight service because of its ability to accelerate tonnage quickly. These locomotives became a familiar sight across the railroad on priority freight and express trains from the late 1920's until the end of steam in 1957. Pennsylvania Railroad No. 6755 was built in 1930 and operated until 1957, when the railroad retired all of its remaining steam locomotives. In 1953, it was rebuilt from class M1A to M1B with a nickel-steel boiler that raised boiler pressure to 270 p.s.i., firebox circulators, and a welded-steel drop-coupler pilot as part of a modernized front end designed to ease maintenance. This locomotive also has an oversized coast-to-coast tender that allowed longer distances between water stops, a dog house on the tender deck where a brakeman monitored the train's status, and a water scoop that let it take on water from a track pan while moving at about 45 miles per hour or less. No. 6755 is the largest surviving Pennsylvania Railroad steam locomotive.