Class M1/M1a (Locobase 220)

Belpaire boiler had feedwater heater, valve motion had limited cutoff. While few American railroads adopted the flat-topped Belpaire-type firebox, the Pennsylvania built most of its steam locomotives with that design and converted other types of boilers. Firebox heating surface included 29 sq ft of arch tubes.

William F. Kiesel, Jr., the road's mechanical engineer, took all earlier 20th Century Pennsy practice -- especially the boiler development represented by the I-1 Decapods -- and added to it a huge combustion chamber ahead of the firebox. (See Locobase 32 for a comment on the unique design of Pennsy's Belpaire firebox.) Locobase has always liked the effect on the M-1s profile of the elongated and angled, square-shouldered firebox.

201 M-1s were built from 1923-1926: 175 by Baldwin, 25 Lima, and 1 at PRR's Juniata works.

100 M-1as were built in 1930: 50 by Baldwin, 25 by Lima, and 25 by Juniata. Larger tenders held 22,090 gal water, 31.5 tons coal.

See Bert Pennypacker's 2-part account of the development and use of these exceptional Mountains in Trains, October and November, 1979. His summary of their reception when they entered service: "The engines ran superbly, were not difficult to fire, and were easy on coal and water. They steamed so well, in fact, that no engineman was ever known to have complained about low steam pressure. But they were dirty hogs to run ..." Most M-1s were hand-fired for most of their careers.

Two significant improvements to the basic design that went into the M1a, were the use of table grates, which allowed much less coal to drop through to the ashpan, and a refashioned front end that included a 6-spoke star nozzle that improved draft, smoother steam flow, and a cleaner exhaust. In 1940, Pennsy designers reduced the superheat surface to 1,550 sq ft.

40 M1as were converted to M1b by increasing the steam pressure to 270 psi, adding 35 sq ft to the fireboxes and fitting them with water circulators. Other M1s had their cast-iron cylinder and half-saddle sections replaced by a welded one-piece saddle and cylinder unit.