According to the late Al Staufer, dean of Pennsy historians, the M-class Mountains were "the most favored and loved class of power on the Pennsylvania Railroad." They were the last and best of the Pennsy's home-grown steamers.Conceived as a dual-purpose passenger and fast freight engine, with the ability to replace L1s Mikados and double-headed K4s Pacifics, the M-Class was developed in typical conservative Pennsy fashion. The boiler design was based largely on the road's I1s 2-10-0's, and its KW trailing truck design was shared by over 1000 Pennsy Atlantics, Pacifics and Mikados.
No. 4700, the M-class prototype, rolled out of the road's Juniata Shops in October, 1923. For more than two years, it was tested, refined, and tested again, both on the road and on the Pennsy's Altoona test plant. The final design was, in the words of author Richard Adams, "pure PRR - a big boiler and big cylinders, and nothing else." Initially, No. 4700 did not even have an automatic coal stoker, until it became evident that no human could keep up with the demands of its huge firebox. What the M-class did have, however, were the qualities that enginemen wanted in a locomotive: smooth riding and free steaming - the ability to never run out of steam, no matter what the demands of load and grade. At full cry, an M-class 4-8-2 could deliver 4000 drawbar horsepower and hustle a train along at 80 mph.
The Pennsy committed to full M-class production in 1926, ordering 175 Class M1 engines from Baldwin Locomotive Works and 25 from Lima Locomotive Works. Four years later, Pennsy upgraded the design to M1a with a new steel cylinder casting, a feedwater heater that improved power, and huge tenders that crews nicknamed the "coast-to-coast" tenders. The 1930 order for 100 M1a's was split between Baldwin, Lima and Juniata.
Beginning in 1946, about 38 M1a engines were upgraded to M1b, with firebox improvements and a higher boiler pressure that increased power even more. Also added was a heavier, fabricated steel pilot that improved front end tracking and featured a drop coupler, to avoid fouling an object in the event of a collision. Our Premier engine is the only die-cast O Gauge model of the M1b.
Initially, the Mountains were assigned to both passenger and freight service, bumping K4s Pacifics from crack passenger runs. Unassisted, an M1 could lift 10 Pullmans over Horseshoe Curve, Pennsy's conquest of the Allegheny Mountains. But in the mid-1930s, as electric power took over mainline work east of Harrisburg, Pennsy's full fleet of passenger K4s' became available to cover less than half the trackage it had been built for. In the midst of the Great Depression, economics dictated that virtually all Mountains be shifted to fast freight service, and it was there they found their true calling. It was not uncommon to see double-headed 4-8-2's speeding along with 125 freight cars in tow. Within a few years, WW II would bring Class M's back into electrified territory, smoking up the wires with a wartime traffic surge.
As Al Staufer put it, "Pennsy hit the jackpot on this one. They were just about the greatest hunk of steam power they ever owned. They were designed with an eye toward passenger and dual service, but they possessed such power that they became the kings of the high speed freight."