The Pennsylvania Railroad's class M1 steam locomotives were a class of heavy mixed-traffic locomotives of the 4-8-2 "Mountain" arrangement, which uses four pairs of driving wheels with a four-wheel guiding truck in front for stability at speed and a two-wheel trailing truck to support the large firebox needed for sustained power. Although built for both passenger and freight work, they spent most of their service lives hauling heavy high-speed freight trains. Many PRR men counted the M1 class locomotives as the best steam locomotives the railroad ever owned. In 1930, a further 100 locomotives were ordered. These, classified as M1a, featured several improvements from the previous locomotives. Instead of separate cylinder block and smokebox saddle castings, the M1a featured a one-piece casting (first seen on the K5 Pacific) which included inside steam delivery pipes, instead of the outside, visible pipes of the M1. The M1a locomotives were fitted as standard with larger tenders than the previous locomotives. The M1a locomotives were intended for passenger as well as freight service but they proved better suited to freight work.