Pennsylvania 2-10-0 "Decapod" Locomotives in the USA

Class Details by Steve Llanso of Sweat House Media

Class I1s Type A (Locobase 5153)

William D. Edson (Keystone Steam & Electric, 1974) adds that these engines also had "[a] Belpaire boiler, [mechanical] stoker, Worthington feedwater heater, power reverse gear, heat-treated steel reciprocating parts, and underhung crossheads." Of the five sets of drivers, only the first and last had flanges, which reduced the design's minimum curve radius. See Locobase 32 for a comment on the unique design of Pennsy's Belpaire firebox.

Pennsy built the first 123 at their Juniata shops in 1916 and 1918-1919. Never very stylish, these were brute-force engines serving wherever there was a long train and a steep grade.

Alco Estimating Engineer James Partington (in "Avoidable Waste in Locomotive Operation as Affected by Design", Railway Age, Volume 95, No. 11 (5 November 1921), pp. 673-677) comments that the secret to the I-1's great success lay in setting the proportions to allow for limited cutoff operation. Continuous high tractive effort levels on the long uphill runs were achieved by using a long stroke and large cylinders, but cutting off the steam at 50% of the stroke rather than the more usual 90%. Limiting the cutoff may allowed the railroad to use 12" (305 mm) piston valves.

"The expected increase in economy of coal and water," Partington observed, "...has been fully realized. Not only has the engine shown remarkable efficiency, but the economy under wide ranges of load is especially remarkable [sic]."

As a reminder of what "efficiency" consisted of in the steam era, note that the I-1 achieved a maximum of 8.1% thermal efficiency (generating 1,777 ihp), and averaged over 7%. Maximum IHP came to 3,080 (at 40% cutoff and 2.9 lb of coal per IHP hour.).

See Locobase 15291 for the 475 Baldwins of 1922-1923, which introduced

Class I1s Type E - 1922 (Locobase 15921)

Data from DeGolyer, Volume 68, pp. 388+ . Works numbers were:


November 55725-55730, 55777-55785

December 55817-55855, 55943, 55945


January 55946-55989, 56069-56076

March 56164-56194

April 56410-56415, 56432-56452

May 56489-56502, 56531-56535, 56546-56565, 56615-56629

June 56643-56682

July 56747-56758, 56776-56803

August 56869-56895, 56945-56967

September 57037-57061, 57100-57104, 57125-57170

October 57229-57231, 57272-57317

William D. Edson (1974) recorded that these engines also had "[a] Belpaire firebox, Worthington [BL-2] feed water heater, [mechanical] stoker, power reverse gear, heat-treated steel reciprocating parts, and underhung crossheads." Of the five sets of drivers, only the first and last had flanges, which reduced the design's minimum curve radius. A combustion chamber measuring 42.25" added 90 sq ft (8.35 sq m), which together with the 31 sq ft (2.9 sq m) of fire brick tubes, completed the supplementary direct heating surface. Also, the Pennsy had adopted an unusually "square" tube and flue arrangement (usually engines of that era fitted with Type E superheaters had many more flues than tubes). See Locobase 32 for a comment on the unique design of Pennsy's Belpaire firebox..

As Locobase 67 relates, tests on the I-1s in 1923 led to changes in the boiler.

Class I1s Type E - 1929 (Locobase 67)

Data from table in 1930 Locomotive Cyclopedia and PRR Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also "Tests of A Class I 1s Freight Locomotive Equipped with a Feed Water Heater and Type E Superheater", Locomotive Testing Plant Bulletin No. 32 (1924). (Many thanks to Chris Hohl for his 22 September 2017 email reporting unlikely boiler pressure values for 177 entries. A Locobase macro caused the error .)

Baldwin delivered these 475 decapods in 1922-1923; see Locobase 15921

Of the five sets of drivers, only the first and last had flanges, which reduced the design's minimum curve radius. See Locobase 32 for a comment on the unique design of Pennsy's Belpaire firebox.

The dimensions shown in the specifications come from the Pennsylvania's diagrams, which were dated from the late 1920s. The changes apparently resulted from the Pennsylvania's tests of an I-1s in February 1923. Engineer of Tests F M Waring stated that the "substitution of the Type E for the Type A superheater and the resultant large increase in heating surface has not noticeably increased the evaporative capacity or efficiency of the boiler." He concludes that the evaporative heating surface area in this design limited any possible gains regardless of the size of the superheater.

Waring also noted that use of the Worthington BL-2 feed water heater resulted in a 14% savings in coal use.

Although no recommendations for modifications to the I-1's boiler appear in the report and the Belpaire firebox remained unchanged, differences found in diagrams prepared just a few years later suggest that the Pennsy's motive power heads decided to adjust the balance of the tube-flue area. Given the Waring's conclusion regarding the boiler's EHS, it's odd that both tube and flue counts both dropped, but the boiler lost more than twice as many 3 1/2" flues than 2 1/4" tubes. Also, tube and flues were all trimmed by 6" (152 mm).

In fact, both the M-1 4-8-2 and the I-1 2-10-0 used boilers with many more small tubes than were found in most boilers with Type E superheaters.

Beginning in 1930, almost all of the I1s engines were redesignated I1sa, which indicated a modification to the valves to permit running at cutoffs of up to 78%. By 1944, 479 had been converted with 119 more still designated I1s.

Principal Dimensions by Steve Llanso of Sweat House Media
ClassI1s Type AI1s Type E - 1922I1s Type E - 1929
Locobase ID5153 15921 67
RailroadPennsylvania (PRR)Pennsylvania (PRR)Pennsylvania (PRR)
Number in Class123425425
Road Numbers
Number Built123425
Valve GearWalschaertWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase (ft / m)22.67 / 6.9122.67 / 6.9122.67 / 6.91
Engine Wheelbase (ft / m)32.17 / 9.8132.17 / 9.8132.17 / 9.81
Ratio of driving wheelbase to overall engine wheebase 0.70 0.70 0.70
Overall Wheelbase (engine & tender) (ft / m)73.04 / 22.2673.04 / 22.2673.37 / 22.36
Axle Loading (Maximum Weight per Axle) (lbs / kg)72,600 / 32,931
Weight on Drivers (lbs / kg)334,500 / 151,727334,500 / 151,727352,500 / 159,892
Engine Weight (lbs / kg)366,500 / 166,242366,500 / 166,242386,100 / 175,132
Tender Loaded Weight (lbs / kg)182,000 / 82,554182,000 / 82,554204,700 / 92,850
Total Engine and Tender Weight (lbs / kg)548,500 / 248,796548,500 / 248,796590,800 / 267,982
Tender Water Capacity (gals / ML)9000 / 34.099000 / 34.0910,300 / 39.02
Tender Fuel Capacity (oil/coal) (gals/tons / ML/MT)17.50 / 15.9017.50 / 15.9018.70 / 17
Minimum weight of rail (calculated) (lb/yd / kg/m)112 / 56112 / 56118 / 59
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)62 / 157562 / 157562 / 1575
Boiler Pressure (psi / kPa)250 / 17.20250 / 17.20250 / 17.20
High Pressure Cylinders (dia x stroke) (in / mm)30.5" x 32" / 775x81330.5" x 32" / 775x81330.5" x 32" / 775x813
Tractive Effort (lbs / kg)102,027 / 46278.72102,027 / 46278.72102,027 / 46278.72
Factor of Adhesion (Weight on Drivers/Tractive Effort) 3.28 3.28 3.45
Heating Ability
Firebox Area (sq ft / m2)272 / 25.28312 / 28.99322 / 29.91
Grate Area (sq ft / m2)70 / 6.5169.90 / 6.4969.90 / 6.50
Evaporative Heating Surface (sq ft / m2)4315 / 401.024818 / 447.604590 / 426.58
Superheating Surface (sq ft / m2)1360 / 126.391986 / 184.501634 / 151.86
Combined Heating Surface (sq ft / m2)5675 / 527.416804 / 632.106224 / 578.44
Evaporative Heating Surface/Cylinder Volume159.46178.05169.62
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation17,50017,47517,475
Same as above plus superheater percentage21,70022,54322,019
Same as above but substitute firebox area for grate area84,320100,620101,430
Power L117,10323,15319,958
Power MT563.61762.98624.11

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Wes Barris