Spokane, Portland & Seattle 4-6-6-4 "Challenger" Locomotives in the USA

The Spokane, Portland & Seattle Railway was jointly owned by the Great Northern and the Northern Pacific Railroads. Most of its steam motive power came to it through purchases of second hand locomotives from its parents. Of the 121 steam locomotives owned by the SP&S throughout its existence only 30 were purchased new and none were of its own design. The eight "Challengers" it owned were new and came as add-ons to Northern Pacific orders.

In 1937 the SP&S needed locomotives and its owners added six 4-6-6-4s to an order placed by the Northern Pacific and in 1944 two more "Challengers" were added to another NP order.

The six "Challengers" received in 1937 were built by the American Locomotive Company and were identical to NP Class Z-6 except they burned oil. On the SP&S they were also designated Class Z-6 and were assigned road numbers 900 through 905. These locomotives had (4) 23 x 32 cylinders, 69" drivers, a 250 psi boiler pressure, exerted 104,500 lbs of tractive effort and weighed 621,000 pounds. Numbers 900 and 901 were sold to the Great Northern Railroad shortly after they arrived in 1937. Number 900 (GN number 4000) returned in 1946 and number 901 (GN number 4001) returned in 1950, both being deemed surplus on the parent railroad.

The two ALCO-built 4-6-6-4s received in 1944 were also oil burners and were designated Class Z-8 and given road numbers 910 and 911. These locomotives had (4) 23 x 32 cylinders, 70" drivers, a 260 psi boiler pressure, a tractive effort of 107,000 lbs and weighed 639,000 pounds.

The eight SP&S "Challengers" were scrapped in 1960.


ClassQty.Road NumberYear Built Builder Notes
Z-6 6900-9051937ALCONumbers 900-905 scrapped in 1960
Z-8 2910-9111944ALCONumbers 910-911 scrapped in 1960

Class Details by Steve Llanso of Sweat House Media

Class Z-6 (Locobase 7574)

Data confirmed by SP&S 2 - 1953 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. Firebox heating surface included 3 thermic syphons. Supposedly the only difference between these locomotives and those of the Northern Pacific was the use of oil for fuel although the firebox heating surface in this sextet totalled about 80 sq ft more. Works numbers were 68990-68995. Also see http://www.spshs.org/res/steam.html

Rick Bolger posted a fascinating account of the role these locomotives played on the Great Northern -- see [], accessed 20 May 2006.

Bolger says that the SP & S was jointly owned by the Northern Pacific and the Great Northern. When the GN gained trackage rights over the Spokane-Wishram and Wishram-Bend (Oregon) sections, a provision allowed them to substitute their own power for the usually provided SP & S engines. The GN and NP got into a wrangle over whether the N-2 2-8-8-0s were of equal capacity to the SP & S Z-6s - of course the NP said no, GN said yes.

"...[F]or the next two years," reports Bolger "the fur (and hot angry memos) flew ...Eventually, the GN gave in, and agreed to purchase two Z-6s ..." These were 903 and 904, according to Bolger, which came to the GN in 1939 and renumbered 4000-4001.

As they had always intended, the GN ran the two between Wishram and Bend. After World War II, they were reassigned (in part, according to Bolger) to the Klamath (Wash) division before being returned to the SP & S in 1947 and 1950, respectively.

The reunited class served until the late 1950s and all 6 went to the Southern Oregon Steel Mills for scrapping on 26 May 1960.

Class Z-8 (Locobase 7575)

Data confirmed by SP&S 2 - 1953 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Rail Data Exchange. Works numbers 71333-71334

The firebox heating surface included 136 sq ft (12.65 sq m) in eight circulators. With roller bearings on all axles and immense grates measuring 246 1/2" long, these engines served heavy freights. See Locobase 338 for a description of the revised bearing design that permitted smooth riding at high speeds.

This pair of Challengers (works #71333-71334) exactly duplicated the 20 supplied to the Northern Pacific except for the use of oil fuel.

Delivered in September 1944, they were sold together to the Southern Oregon Steel Mills for scrapping on 25 May 1960.

Principal Dimensions by Steve Llanso of Sweat House Media
Locobase ID7574 7575
RailroadSpokane, Portland & SeattleSpokane, Portland & Seattle
Number in Class62
Road Numbers900-905910-911
Number Built62
Valve GearWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase (ft / m)24.34 / 7.4224.67 / 7.52
Engine Wheelbase (ft / m)61.83 / 18.8562.17 / 18.95
Ratio of driving wheelbase to overall engine wheebase 0.39 0.40
Overall Wheelbase (engine & tender) (ft / m)113.67 / 34.65110.04 / 33.54
Axle Loading (Maximum Weight per Axle) (lbs / kg)73,400 / 33,29474,000 / 33,566
Weight on Drivers (lbs / kg)430,000 / 195,045444,000 / 201,395
Engine Weight (lbs / kg)621,000 / 281,681644,000 / 292,114
Tender Loaded Weight (lbs / kg)410,000 / 185,973437,000 / 198,220
Total Engine and Tender Weight (lbs / kg)1,031,000 / 467,6541,081,000 / 490,334
Tender Water Capacity (gals / ML)20,000 / 75.7625,000 / 94.70
Tender Fuel Capacity (oil/coal) (gals/tons / ML/MT)6000 / 22.706500 / 24.60
Minimum weight of rail (calculated) (lb/yd / kg/m)119 / 59.50123 / 61.50
Geometry Relating to Tractive Effort
Driver Diameter (in / mm)69 / 175370 / 1778
Boiler Pressure (psi / kPa)250 / 17.20260 / 17.90
High Pressure Cylinders (dia x stroke) (in / mm)23" x 32" / 584x813 (4)23" x 32" / 584x813 (4)
Tractive Effort (lbs / kg)104,267 / 47294.77106,888 / 48483.64
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.12 4.15
Heating Ability
Firebox Area (sq ft / m2)839 / 77.95756 / 70.23
Grate Area (sq ft / m2)152 / 14.13152 / 14.13
Evaporative Heating Surface (sq ft / m2)5832 / 542.015749 / 534.29
Superheating Surface (sq ft / m2)2114 / 196.472105 / 195.63
Combined Heating Surface (sq ft / m2)7946 / 738.487854 / 729.92
Evaporative Heating Surface/Cylinder Volume189.50186.80
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation38,00039,520
Same as above plus superheater percentage48,26050,190
Same as above but substitute firebox area for grate area266,383249,631
Power L127,14028,064
Power MT834.88836.09

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