Western Pacific 4-6-6-4 "Challenger" Locomotives in the USA

The Western Pacific Railroad purchased seven 4-6-6-4s from the American Locomotive Company in 1938. These "Challengers" were designated Class M-100 and were assigned road numbers 401 through 407. They had four 22" x 32" cylinders, 70" drivers, a 265 psi boiler pressure, exerted 99,600 lbs of tractive effort and weighed 590,000 pounds.

The Western Pacific used its "Challengers" for freight service across the desert between Elko, NV and Salt Lake City, UT until they were retired in 1952.

There are no surviving Western Pacific "Challengers".


ClassQty.Road NumberYear Built Builder Notes
M-100 7401-4071938ALCONumbers 410-407 scrapped in 1952

Class Details by Steve Llanso of Sweat House Media

Class M-100 (Locobase 343)

Data from WP-2 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. See also the 1947 Locomotive Cyclopedia and S Kip Farrington, Jr, Railroading Coast to Coast (New York: Bonanza Books, 1976), p. 123. Roster information from Frank Brehm's compilation hosted at [] (accessed 3 July 2006). Works numbers were 68902-68908 in January 1938.

Little-remarked group of Challengers for the WP that ran across the desert from Elko, Nevada to Salt Lake City, Utah. The data convey in many ways just how big a Challenger truly was. The Western Pacific's diagram book contains a few more: boiler water capacity with 1 gauge of water was 7,750 US gallons (29,334 litres). 2 gauges full equated to 8,310 gallons (31,453 litres), and 3 gauges meant 8,840 US gallons (33,459 litres). Locobase can't resist comparing those oceans of water to a swimming pool and calculates that a 3-gauge boiler would fill a oval pool 40 feet long, 8 feet wide, and 9 feet deep (12.2 x 2.44 x 2.74 m).

Four thermic syphons contributed 151 sq ft (14.03 sq m) to the direct heating surface. Each cylinder was fed by its own 12" (305 mm) piston valve. The four wheels of the leading truck turned ASF roller bearings, while the six-wheel trailing truck used SKF rollers.

Very similar to the Clinchfield and D&H locomotives running on this wheel arrangement. See Locobase 338 for a description of the revised bearing design that permitted smooth riding at much higher speeds than usually reached by articulated locomotives. Even before that update, S Kip Farrington, Jr's note in a 29 May 1940 engine log said the 402 hauled 57 cars aggregating 2,780 tons (2,527 tonnes) "extremely well taking the Wendover hill, 68 miles (109 km) of 1%, with ease."

Each of these engines cost $181,762 on delivery in 1938. Fourteen years later, supplanted by diesels, their scrap value amounted to $13,762 apiece in May and June 1952.

Principal Dimensions by Steve Llanso of Sweat House Media

Locobase ID343
RailroadWestern Pacific (WP)
Number in Class7
Road Numbers401-407
Number Built7
Valve GearWalschaert
Locomotive Length and Weight
Driver Wheelbase (ft / m)24.34 / 7.42
Engine Wheelbase (ft / m)59.92 / 18.26
Ratio of driving wheelbase to overall engine wheelbase 0.41
Overall Wheelbase (engine & tender) (ft / m)106.54 / 32.47
Axle Loading (Maximum Weight per Axle) (lbs / kg)66,500 / 30,164
Weight on Drivers (lbs / kg)399,000 / 180,984
Engine Weight (lbs / kg)590,000 / 267,620
Tender Loaded Weight (lbs / kg)401,500 / 182,118
Total Engine and Tender Weight (lbs / kg)991,500 / 449,738
Tender Water Capacity (gals / ML)22,000 / 83.33
Tender Fuel Capacity (oil/coal) (gals/tons / Liters/MT)25 / 23
Minimum weight of rail (calculated) (lb/yd / kg/m)111 / 55.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)70 / 1778
Boiler Pressure (psi / kPa)265 / 18.30
High Pressure Cylinders (dia x stroke) (in / mm)22" x 32" / 559x813 (4)
Tractive Effort (lbs / kg)99,676 / 45212.33
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.00
Heating Ability
Tubes (number - dia) (in / mm)222 - 2.25" / 57
Flues (number - dia) (in / mm)60 - 5.5" / 140
Flue/Tube length (ft / m)23 / 7.01
Firebox Area (sq ft / m2)692 / 64.29
Grate Area (sq ft / m2)108.20 / 10.06
Evaporative Heating Surface (sq ft / m2)5666 / 526.58
Superheating Surface (sq ft / m2)1724 / 160.22
Combined Heating Surface (sq ft / m2)7390 / 686.80
Evaporative Heating Surface/Cylinder Volume201.21
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation28,673
Same as above plus superheater percentage35,268
Same as above but substitute firebox area for grate area225,557
Power L127,054
Power MT896.90

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