M1 was the class designation used for the Pennsylvania Railroad's 4-8-2 (or "Mountain") locomotives. The M1 prototype, #4700, was built by the railroad's Altoona shops in 1923. Many design features were carried over from the PRR's I1s class 2-10-0 of 1916, which had proved immensely successful in slow freight service.

The driving wheels of the M1 were 72" in diameter, which permitted the locomotives to be used in either passenger or fast freight service. After three years of testing, an order for 200 more M1 types was placed in 1926. These locomotives carried #'s 6800-6999. Most were home-built at Altoona. A further 100 engines, of a slightly improved design known as M1a, were built in 1930. These engines were numbered 6700-6799, and the original M1 was renumbered to 6699. The only visible difference between the M1 and the M1a was that the M1a had internal steam-delivery pipes to the cylinders, while the M1's were external.

The M1 and M1a types did not last long in passenger service. The PRR owned, at this time, over 600 4-6-2 types of classes K2s, K3s, and K4s, and by the mid-1930s, the GG1 electrics handled the passenger traffic east of Harrisburg, Pennsylvania. The M1 also rode more roughly than the K4s "Pacific", and were unable to handle the longer Limited trains without doubleheading (in all fairness, the passenger locomotive that could pull a 20-car passenger train around Horseshoe Curve by itself may never have been invented). The demand for them was greater in freight service, where they were the only freight locomotive rated at a speed greater than 50 mph.

The M1 developed about 65,000 pounds of tractive effort, with a steam pressure of 250 pounds. It was an extremely successful design, and remained the PRR's premier freight power until the arrival of General Motors F7 and GP7 diesels in the late 1940s. Some of the M1 types were downgraded to slow freight and helper service, while a handful of M1a types received a rebuild that included a further increase in steam pressure, and were re-classed M1b. By 1955-1956, the M1 and M1a engines were being retired en masse. By 1957, only 15 were still operating on PRR's Central Division, with another 30 stored serviceable.

M1b #6755 has been preserved at the Railroad Museum of Pennsylvania, located in Strasburg, Pennsylvania.