Pennsylvania 4-8-2 "Mountain" Type Locomotives

Introduction

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 feedwater heaters.

There is one PRR Mountain survivor, number 6755, at the Railroad Museum of Pennsylvania in Strasburg, PA.

Roster by Richard Duley

ClassQtyRoad NumbersYear BuiltBuilder
M1166991923PRR
M11756800 - 69741926Baldwin
M1256975 - 69991926Lima
M1a506700 - 67491930Baldwin
M1a256750 - 67741930PRR
M1a256775 - 67991930Lima

Class Details by Steve Llanso

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 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 ..."

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 6-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.


Specifications by Steve Llanso
ClassM1M1a
Locobase ID220 14257
RailroadPennsylvania (PRR)Pennsylvania (PRR)
CountryUSAUSA
Whyte4-8-24-8-2
Road Numbers4700, 6800-69996700-6799
GaugeStdStd
BuilderseveralSeveral
Year19231930
Valve GearWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase18.83'18.83'
Engine Wheelbase41.04'41.80'
Ratio of driving wheelbase to overall engine wheebase 0.46 0.45
Overall Wheelbase (engine & tender)79.32'79.32'
Axle Loading (Maximum Weight per Axle)72500 lbs70000 lbs
Weight on Drivers267000 lbs271000 lbs
Engine Weight385850 lbs390000 lbs
Tender Light Weight221050 lbs378360 lbs
Total Engine and Tender Weight606900 lbs768360 lbs
Tender Water Capacity11980 gals22020 gals
Tender Fuel Capacity (oil/coal)18.5 tons31.5 tons
Minimum weight of rail (calculated) on which locomotive could run111 lb/yard113 lb/yard
Geometry Relating to Tractive Effort
Driver Diameter72"72"
Boiler Pressure250 psi250 psi
Cylinders (dia x stroke)27" x 30"27" x 30"
Tractive Effort64547 lbs64547 lbs
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.14 4.20
Heating Ability
Firebox Area397 sq. ft397 sq. ft
Grate Area69.90 sq. ft69.90 sq. ft
Evaporative Heating Surface4700 sq. ft4702 sq. ft
Superheating Surface1632 sq. ft1630 sq. ft
Combined Heating Surface6332 sq. ft6332 sq. ft
Evaporative Heating Surface/Cylinder Volume236.41236.51
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation1747517475
Same as above plus superheater percentage2201922019
Same as above but substitute firebox area for grate area125055125055
Power L13225232228
Power MT1065.221048.72

Photos

Reference

Credits

Introduction and roster provided by Richard Duley. Class details and specifications provided by Steve Llanso of Sweat House Media.