The Pennsylvania Railroad's Juniata Shops in Altoona, PA, outshopped a single
"Mountain" 4-8-2 locomotive in 1923. It was assigned road number 4700 (later
is was renumbered 6699) and was designated Class M1. It was a common practice
for the PRR to build a prototype that could be tested thoroughly before
ordering more of a locomotive design. It tested the Class M1 prototype for
two years before approving it for fleet construction.
The design of the PRR's "Mountain" locomotive was not an extension of its
Class K4s "Pacific", but rather a descendant of its I1s "Decapod". It was
the work of J. T. Wallis, Chief of Motive Power and W. F. Kiesel, Jr,
Mechanical Engineer, who started with an I1's boiler, enlarged the combustion
chamber, then used the "Decapod's" guides, crossheads and many other parts.
In 1925, the PRR ordered 200 Class M1 "Mountains", 175 (road numbers 6800
through 6974) from the Baldwin Locomotive Works and 25 (road numbers 6975
through 6999) from the Lima Locomotive Company. These locomotives had 72"
drivers, 27 x 30 cylinders, a 250 psi boiler pressure, a tractive effort of
64,550 lbs and weighed 385,000 pounds.
One hundred, Class M1a "Mountains" were added to the roster in 1930, fifty
(road numbers 6700 through 6749) from Baldwin, twenty (road numbers 6777
through 6799) from Lima and twenty (road numbers 6750 through 6774) built in
the Juniata Shops. Class M1a locomotives were very similar to the Class M1s
but, included two air compressors instead of one and had Worthington
There is one PRR Mountain survivor, number 6755, at the Railroad Museum
of Pennsylvania in Strasburg, PA.
Class M1 (Locobase 220)
Data from table and diagram in 1930 Locomotive Cyclopedia and PRR Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection. See also DeGolyer, Vol 77, pp. 436. (Thanks to Chris Hohl for his 20 Feb 2013 email pointing out the absurdity of using the tender weight for the M1's engine weight and for a later correction on the road number for this class) See Locobase 14257 for the 1930-built M1a.
201 M-1s were built from 1923-1926: 175 by Baldwin (works numbers were 59340,
59357-59369 in July 1926; 59407-59435 in August; 59460-59469, 59478-59522 in September; 59557-59604 in October; 59655-59683 in November),
25 Lima (works numbers 7099-7113 in September 1926 and 7114-7123 in October 1926), and 1 at PRR's Juniata works.
Valve motion had limited cutoff and operated 12" (305 mm) diameter piston valves. 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 (2.7 sq m) of arch tubes.
William F. Kiesel, Jr., the road's mechanical engineer, took all earlier 20th Century Pennsylvania practice -- especially the boiler development represented by the I-1 Decapods -- and added to it a huge combustion chamber ahead of the firebox that contributed 164 sq ft (15.2 sq m) to the firebox heating surface area. (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. Fred Westing, in his The Locomotives that Baldwin Built (Seattle, Wash: Superior Publishing, 1966), p. 135, says this long pair of cavities were joined by corrugated plates at top and bottom to allow expansion. The top plate had 2" radius corrugations and formed a 5" deep trough across the crown sheet. The bottom plate was crescent-shape and had a maximum depth of 2".
Although the builders hadn't quite achieved the integrally cast frame, the cast frames on each side were substantial members, says Westing, with a maximum width of 7 inches (179 mm) "throughout the greater portion of their length" and 9 1/2" (241 mm) above the driving axle pedestals. They were braced by "equally massive" transverse beams.
See Bert Pennypacker's two-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 ..."
Westing cites an example of their power at speed in the hauling of 125 loaded cars (4,200 tons) up the grades of the Middle Division in about four hours. Coming east over the same division, the M1s could pull 140 loaded coal cars.
See the K4s entry (Locobase 159) for the "bank-firing" technique that K4 firemen found useful for long-distance run. It's not clear if M1s were able to use the technique, which worked best when the locomotive was unlikely to make a long stop during the trip.
Class M1a (Locobase 14257)
Data from table and diagram in 1930 Locomotive Cyclopedia and PRR Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive collection. See also DeGolyer, Vol 80, pp.1+ . (Thanks to Chris Hohl for spotting the numbering discrepancy in this class.)
100 M-1as were built in 1930: 50 by Baldwin (works numbers were 61243-61246, 61267-61268 in March; 61294-61303, 61310-61311 in April; 61334-61349, 61362-61369 in May; 61402-61407 in June; 61447-61448 in August;
25 by Lima (works numbers were 7443-7453, in March 1930, 7454-7465 in April and 7466-7467 in May), and 25 by Juniata.
See Locobase 220 for a more complete discussion of the M1 design. In both variants, the valve motion had limited cutoff and operated 12" (305 mm) diameter piston valves. 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 (2.7 sq m) of arch tubes.
Three significant improvements to the basic design resulted in the M1a. One was the use of table grates, which allowed much less coal to drop through to the ashpan. A refashioned front end included a 6-spoke star nozzle that improved draft, smoother steam flow, and a cleaner exhaust. And a Worthington feedwater heater raised boiler efficiency at the small expense of a 9" (229 mm) longer engine wheelbase and 5,000 lb (2,268 kg) weight increase to 390,000 lb (176,901 kg). Also, the Pennsy combined these with the enormous six-axle P75/F75 "coast-to-coast" tender which was--at last!--equipped with an automatic stoker.
In 1940, Pennsy designers reduced the superheat surface to 1,550 sq ft.
40 M1as were converted to M1b in 1946 by increasing the steam pressure to 270 psi, adding 35 sq ft (3.25 sq m) 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.