Northern Pacific 4-6-6-4 "Challenger" Locomotives of the USA

The Northern Pacific ordered 21 "Challengers" from the American Locomotive Company in 1936 and took delivery of them during 1936 and 1937. These 4-6-6-4s were designated Class Z-6 and were assigned road numbers 5100 through 5120. They had four 23 x 32 cylinders, 69" drivers, a 250 psi boiler pressure, exerted 104,500 lbs of tractive effort and weighed 624,500 pounds.

In 1941, another six "Challengers" came from ALCO. These were designated Class Z-7 and they carried road numbers 5121 through 5126. They had four 23 x 32 cylinders, 70" drivers, a 260 psi boiler pressure, exerted 107,000 lbs of tractive effort and weighed 644,000 pounds.

The last steam locomotives bought by the Northern Pacific were the 20 "Challengers" it purchased from ALCO during 1943 and 1944. This group designated Class Z-8 was assigned road numbers 5130 through 5149. The Class Z-8 locomotives were virtually identical those of Class Z-7. Two oil-burning Z-8s were also delivered to the SP&S.

There are no surviving Northern Pacific "Challengers".

Roster by Richard Duley

ClassQty.Road NumberYear Built Builder Notes
Z-6125100-51111936ALCONumbers 5100-5111 scrapped in the early 1950s
Z-695112-51201937ALCONumbers 5112-5120 scrapped in the early 1950s
Z-765121-51261941ALCONumbers 5121-5126 scrapped in the early 1950s
Z-8105130-51391943ALCONumbers 5130-5139 scrapped in the early 1950s
Z-8105140-51491944ALCONumbers 5140-5149 scrapped in the early 1950s

Class Details by Steve Llanso of Sweat House Media

Class Z-6 (Locobase 927)

Data confirmed by locomotive diagrams from 1900 hosted on (7 February 2004). (Thanks to Chris Hohl for his email noting a discrepancy in road numbers and tender water and fuel capacities.) Works numbers were 68760-68780 in 1936-1937. Average cost for this class was $183,588.

Firebox heating surface included three thermic syphons. All four cylinders received their steam through 12" (305 mm) piston valves. The front pair of valves had 7 1/2" (191 mm) travel, the rear set 7" (178 mm).

The NP diagram gives the dimensions of this engine's cavernous firebox: 246 1/8" (6.25 m) long, 114 1/4" (2.9 m) wide at the bottom. Height at the front was 89 5/8"(2.28 m), at the back, 78 1/2" (2.0 m); only quite tall basketball players would have to duck their heads. At the front top edge, the firebox spanned 87"(2.21 m), tapering only slightly to 85 1/2" (2.17 m) at the back.

The Mainstreeter of Winter 1999 had a long interview with long-time engineer Warren McGee-- (preserved online at, accessed most recently on 30 Jan 2006) -- in which he commented on the hunger and thirst of such a large locomotive. Speaking of the low-calorie Rosebud coal the railroad used, he said: " It was a coal that on the ...Z-6s, you had about an hour and 50 minutes wide open, or two hours and 20, it depended on the firing capabilities of the fireman, and the steaming qualities of the engine."

Note that the first four Z-6s were delivered with friction bearings on the drivers; they weighed 4,500 lb less (430,500 lb) than the later engines, which used roller bearings. The latter weight is the figure given in the specifications.

The class remained in service well into the 1950s. 5105, 5110 went first, being scrapped in October 1954. 5108 followed in December 1955. 5103 went in April 1957; 5100, 5109, and 5111 in September; and 5102, 5104, 5106-5107 completed the process in April 1958 (5101 was scrapped sometime in 1958.)

Class Z-7 (Locobase 340)

Data from tables and diagrams in 1947 Locomotive Cyclopedia, supplemented by NP TO 1944 Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive Raildata collection. Works numbers were 69540-69545 in 1941

Firebox had eight circulators, a grate length of 192 in. Notice that even with a combustion chamber extending 7 feet 5 inches into the boiler, the tubes flues still measured 23 feet in length. Fitted with Timken roller bearings on all axles on both engine and tender.

Used for heavy freights over steep grades. Note the size of the grate that burned low-calorie "Rosebud coal."

The Z-8 class delivered in 1943 has its own entry at Locobase 928.

Class Z-8 (Locobase 928)

Data from NP to 1944 Locomotive Diagram book supplied in May 2005 by Allen Stanley from his extensive Raildata collection.

This class was essentially identical to the 1941 Z-7s described in Locobase 340, but were produced under a wartime mandate to replace certain high-strength steel components with similar items produced with more common and heavier steel. Even so, the NP's diagrams show no difference in the adhesion weight and just a 1,000 lb (454 kg) difference on engine weight.

The firebox heating surface included 136 sq ft (12.63 m) in eight circulators. See Locobase 338 for a description of the revised bearing design that permitted smooth riding at higher speeds.

The Mainstreeter of Winter 1999 had a long interview with long-time engineer Warren McGee, (preserved online at, accessed most recently on 30 Jan 2006), in which he noted that on the run between Bozeman and Townsend, "I've swept the tank with the broom to get every last drop of coal out of that tender ...Townsend to Bozeman was 27 tons of coal every time, run through that engine."

McGee explained that the grades had something to do with it: "It's all climbing. You'd never shut off, Logan to Bozeman. Townsend to Logan [the other direction], why you had to shut off and avoid going around those curves too fast, so you didn't work full throttle all the way through there. You had to conserve all the coal you could if you didn't want to stop at Logan."

5140 and 5148 were converted to oil burners.

Specifications by Steve Llanso of Sweat House Media
Locobase ID927 340 928
RailroadNorthern Pacific (NP)Northern Pacific (NP)Northern Pacific (NP)
Number in Class21620
Road Numbers5100-51205121-51265130-5149
Number Built21620
Valve GearWalschaertWalschaertWalschaert
Locomotive Length and Weight
Driver Wheelbase12.17'12.33'12.33'
Engine Wheelbase61.83'62.17'62.17'
Ratio of driving wheelbase to overall engine wheebase 0.20 0.20 0.20
Overall Wheelbase (engine & tender)110.04'
Axle Loading (Maximum Weight per Axle)72850 lbs74000 lbs74000 lbs
Weight on Drivers435000 lbs444000 lbs444000 lbs
Engine Weight624500 lbs643000 lbs644000 lbs
Tender Light Weight398400 lbs438000 lbs437000 lbs
Total Engine and Tender Weight1022900 lbs1081000 lbs1081000 lbs
Tender Water Capacity22000 gals25000 gals25000 gals
Tender Fuel Capacity (oil/coal)27 tons27 tons27 tons
Minimum weight of rail (calculated)121 lb/yard123 lb/yard123 lb/yard
Geometry Relating to Tractive Effort
Driver Diameter69"70"70"
Boiler Pressure250 psi260 psi260 psi
Cylinders (dia x stroke)23" x 32"23" x 32"23" x 32"
Tractive Effort104267 lbs106888 lbs106888 lbs
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.17 4.15 4.15
Heating Ability
Firebox Area756 sq. ft756 sq. ft756 sq. ft
Grate Area152 sq. ft152.30 sq. ft152 sq. ft
Evaporative Heating Surface5832 sq. ft5749 sq. ft5749 sq. ft
Superheating Surface2114 sq. ft2105 sq. ft2105 sq. ft
Combined Heating Surface7946 sq. ft7854 sq. ft7854 sq. ft
Evaporative Heating Surface/Cylinder Volume189.50186.80186.80
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation380003959839520
Same as above plus superheater percentage482605028950190
Same as above but substitute firebox area for grate area240030249631249631
Power L1267532806428064
Power MT813.52836.09836.09


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