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.
|Class||Qty||Road Numbers||Year Built||Builder|
|M1||175||6800 - 6974||1926||Baldwin|
|M1||25||6975 - 6999||1926||Lima|
|M1a||50||6700 - 6749||1930||Baldwin|
|M1a||25||6750 - 6774||1930||PRR|
|M1a||25||6775 - 6799||1930||Lima|
Data from table and diagram in 1930 Locomotive Cyclopedia and PRR Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also DeGolyer, Volume 77, pp. 436; and Bert Pennypacker, Atterbury's Engines (2 parts), "Great increases in horsepower and speed,", Trains (October 1979), pp. 22-32 and "She could get on her knees and work tonnage, her 72-inch drivers notwithstanding", Trains, (November 1979), pp. 44-49 . (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. Tests of a Worthington feed water heater revealed a tendency to send water into the cylinders when operating at high rates, so it was deleted from the design. On the other hand, a long-standing resistance to the use of an automatic stoker yielded to greater demand. So the production engines used a Dupont-Simplex Type B from Standard Stoker for the first 75 locomotives and a Duplex Type D-1 from Locomotive Stoker Company. Bert Pennypacker summarized the M1s' 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. 60 M1s served the Harrisburg-Altoona section with its ruling grade of 1.86%, 32 handled traffic in the Philadelphia Division east of Harrisburg, and the remainder operated between Crestline, Ohio and Pittsburgh or on the Fort Wayne or St Louis Division in the West. 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.
Data from table and diagram in 1930 Locomotive Cyclopedia and PRR Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also DeGolyer, Volume 82, pp.1+; and Bert Pennypacker, Atterbury's Engines (Part 2), "She could get on her knees and work tonnage, her 72-inch drivers notwithstanding", Trains, (November 1979), pp. 44-49; and Paul T Warner, "Mountain Type Locomotives on the Pennsylvania", Baldwin Locomotives, Volume 11, No. 3 (January 1933), pp. 3-14. (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. Of Baldwin's 50 engines,the first ten were intended for passenger service, the latter 40 for freight. 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 six-spoke star nozzle that improved draft, smoothed steam flow, and produced a cleaner exhaust. The iron cylinders cast with a half saddle in the M1s were replaced by cast-steel cylinders cast integrally with the full saddle and the center pin for the front truck. The redesign meant that the large steam pipes exiting the boiler barrel to feed the piston valves in M1 locomotives were housed within the boiler in M1as and allowed easy distinction between the two subclasses. A Worthington feed water 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). The effect of all of this attention to steam production, according to Warner, was demonstrated by 6707's run over the 130.8 miles (211 km) eastbound from Altoona to Harrisburg. The engine pulled its 16 car, 1,100-ton train at an average speed of 50 mph (81 kph) in an easy run during which the stoker fired the grate and the feed water heater supplied all of the boiler's water. "With such a fast schedule," commented Warner,"and with speed restrictions on the many sharp curves at the western end of the division, it was necessary to cover long stretches at speeds of 65 to 70 miles per hour [105-113 kph]." Also, the Pennsy combined these with the enormous six-axle P75/F75 "coast-to-coast" tender which was equipped with an updated Dupont-Simplex Type B stoker from Standard Stoker. Modifications included a 6" (152 mm) increase in the length of the front conveyor trough and [Archimedian] screw and, said Baldwin's specs, "showing the Standard Stoker Co.'s latest design of protecting grates around the vertical housing." The 10 M1as assigned to passenger service joined 20 M1s. Most operated trains over the 435.5 miles (701 km) between Harrisburg and Columbus, Ohio. An M1 would either run through without change or be turned at Pittsburgh, 244.6 miles (394 km) west of Harrisburg. In his 1933 report, Warner said that a westbound train would climb the 1.86% grade pulling ten cars without a helper at an average speed of 30 mph (48 kph). Eastbound, a ruling grade of 1% allowed a single M1 to pull 16-18 cars. Freights over moderate grades drew most of the M1a engines. 41 served the Philadelphia Division east of Harrisburg. In 1940, Pennsy designers reduced the superheat surface to 1,550 sq ft (144 sq m).. 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.
|Principal Dimensions by Steve Llanso of Sweat House Media|
|Railroad||Pennsylvania (PRR)||Pennsylvania (PRR)|
|Number in Class||201||100|
|Road Numbers||4700, 6800-6999||6700-6799|
|Locomotive Length and Weight|
|Driver Wheelbase (ft / m)||18.83 / 5.74||18.83 / 5.74|
|Engine Wheelbase (ft / m)||41.04 / 12.51||41.80 / 12.74|
|Ratio of driving wheelbase to overall engine wheebase||0.46||0.45|
|Overall Wheelbase (engine & tender) (ft / m)||79.32 / 24.18||79.32 / 24.18|
|Axle Loading (Maximum Weight per Axle) (lbs / kg)||72,500 / 32,885||70,000 / 31,752|
|Weight on Drivers (lbs / kg)||267,000 / 121,109||271,000 / 122,924|
|Engine Weight (lbs / kg)||385,850 / 100,267||390,000 / 176,901|
|Tender Loaded Weight (lbs / kg)||221,050 / 100,267||378,360 / 171,621|
|Total Engine and Tender Weight (lbs / kg)||606,900 / 200,534||768,360 / 348,522|
|Tender Water Capacity (gals / ML)||11,980 / 45.38||22,020 / 83.41|
|Tender Fuel Capacity (oil/coal) (gals/tons / ML/MT)||18.50 / 16.80||31.50 / 28.60|
|Minimum weight of rail (calculated) (lb/yd / kg/m)||111 / 55.50||113 / 56.50|
|Geometry Relating to Tractive Effort|
|Driver Diameter (in / mm)||72 / 1829||72 / 1829|
|Boiler Pressure (psi / kPa)||250 / 17.20||250 / 17.20|
|High Pressure Cylinders (dia x stroke) (in / mm)||27" x 30" / 686x762||27" x 30" / 686x762|
|Tractive Effort (lbs / kg)||64,547 / 29278.06||64,547 / 29278.06|
|Factor of Adhesion (Weight on Drivers/Tractive Effort)||4.14||4.20|
|Firebox Area (sq ft / m2)||397 / 36.88||397 / 36.88|
|Grate Area (sq ft / m2)||69.90 / 6.49||69.90 / 6.49|
|Evaporative Heating Surface (sq ft / m2)||4700 / 436.64||4702 / 436.83|
|Superheating Surface (sq ft / m2)||1632 / 151.62||1630 / 151.43|
|Combined Heating Surface (sq ft / m2)||6332 / 588.26||6332 / 588.26|
|Evaporative Heating Surface/Cylinder Volume||236.41||236.51|
|Computations Relating to Power Output (More Information)|
|Robert LeMassena's Power Computation||17,475||17,475|
|Same as above plus superheater percentage||22,019||22,019|
|Same as above but substitute firebox area for grate area||125,055||125,055|