Northern Pacific 4-8-0 "Mastodon" Locomotives of the USA

Class Details by Steve Llanso of Sweat House Media

Class X (Locobase 929)

Some data from TraitT pratique de la machine locomotive ... By Maurice Demoulin, 1898

Librairie polytechnique, Baudry et Cie, p 404-405

These 12-wheelers were delivered as compounds in 1897 and intended, said Demoulin, for the Helena-Missoula section. This portion of the NP had a ruling grade of 2.2% that covered almost 17 miles. The writer noted a horsepower output of 1,300 continuous over a 3-hour period. Although regarded as mammoths of their day when they first appeared, this quartet would soon be outstripped in their dimensions on the NP by Schenectady-built Ten-wheelers (Locobase 9542).

Rebuilt 1917 with two 20" x 30" cylinders and a superheater, TE of 32,454. See Locobase 6561.

NB: The evaporative heating surface is an estimate calculated by adding the calculated tube heating surface to the reported firebox heating surface.

Class X - superheated (Locobase 6561)

Data from a 1944 Northern Pacific Locomotive Diagram book supplied in May 2005 by Allen Stanley from his extensive Raildata collection. Locobase is intrigued by all this attention paid to a rarely used wheel arrangement: superheater, piston valves, new valve gear, new tubes and flues ... The later diagram describes the Xs as "switchers" and it's possible that they filled a key niche in short-haul freight.

Specifications by Steve Llanso of Sweat House Media
ClassXX - superheated
Locobase ID929 6561
RailroadNorthern Pacific (NP)Northern Pacific (NP)
Number in Class44
Road Numbers13-1613-16
Number Built4
Valve GearStephensonWalschaert
Locomotive Length and Weight
Driver Wheelbase15.50'15.50'
Engine Wheelbase26.33'26.33'
Ratio of driving wheelbase to overall engine wheebase 0.59 0.59
Overall Wheelbase (engine & tender)
Axle Loading (Maximum Weight per Axle)40900 lbs
Weight on Drivers150000 lbs146200 lbs
Engine Weight186000 lbs184200 lbs
Tender Light Weight94000 lbs94000 lbs
Total Engine and Tender Weight280000 lbs278200 lbs
Tender Water Capacity4350 gals4350 gals
Tender Fuel Capacity (oil/coal)9 tons9 tons
Minimum weight of rail (calculated)63 lb/yard61 lb/yard
Geometry Relating to Tractive Effort
Driver Diameter55"55"
Boiler Pressure200 psi175 psi
High Pressure Cylinders (dia x stroke)23" x 30"20" x 30"
Low Pressure Cylinders (dia x stroke)34" x 30" (1)
Tractive Effort33653 lbs32455 lbs
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.46 4.50
Heating Ability
Firebox Area206.50 sq. ft206.50 sq. ft
Grate Area35 sq. ft35.05 sq. ft
Evaporative Heating Surface2764 sq. ft2234 sq. ft
Superheating Surface468 sq. ft
Combined Heating Surface2764 sq. ft2702 sq. ft
Evaporative Heating Surface/Cylinder Volume383.19204.80
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation70006134
Same as above plus superheater percentage70007176
Same as above but substitute firebox area for grate area4130042281
Power L1441610998
Power MT259.62663.38


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