Great Northern's 4-6-6-4 "Challengers"

Published: August 9, 2025

The Great Northern was well known for operating numerous large articulated steam engines including 2-6-6-2s, 2-8-8-0s, and 2-8-8-2s. In addition, it utilized 4-8-4s and 2-10-2s. However, the railroad maintained only a small roster of 4-6-6-4s.

It ultimately utilized only two examples and these were acquired second-hand from subisidary Spokane, Portland & Seattle in 1937 - not long after the engines had arrived from American Locomotive.

On the GN, the two locomotives were numbered 4000-4001 (ex-SP&S #900-901) where they operated largely in Washington and Oregon. Ultimately, they were returned to the Spokane road between 1946 (900) and 1950 (901).

A Brief History Of GN

The Great Northern was officially incorporated in 1889, evolving from the St. Paul, Minneapolis & Manitoba Railway, which Hill had acquired in 1878. Hill's ambition was to create a northern transcontinental route that would efficiently serve expanding agricultural and industrial markets.

The completed mainline stretched from St. Paul, Minnesota, to Seattle, Washington, with construction finishing in January 1893. This made the Great Northern the first transcontinental railroad in the U.S. built without public subsidies.

The route was strategically designed to pass through regions rich in natural resources, including timber and minerals, while also fostering settlement and agricultural development along the way.

The cities of Spokane, Washington, and Minneapolis-St. Paul became crucial nodes in this expanding network. Hill's approach prioritized gradual expansion, building only where potential profit could be realized, and incorporating innovative engineering practices to navigate the challenging terrain of the Rocky Mountains and Cascades.

The Great Northern was notable for its efficiency and financial stability compared to other railroads of the era, many of which were plagued by debt and mismanagement. Hill's emphasis on superior track quality, efficient operations, and customer service distinguished the GNR from its competitors. Furthermore, Hill was instrumental in forging trade links with Asia, particularly in timber, coal, and wheat exports via the Pacific, bolstering the economic integration of the western U.S. with global markets.

Throughout the early to mid-20th century, the Great Northern expanded and modernized, including purchasing new locomotives and investing in passenger services like the famous Empire Builder, which was named in honor of Hill himself. However, changing transportation trends and the rise of automobiles and airlines gradually reduced passenger numbers throughout the industry.

In 1970, GN merged with subsidaries Northern Pacific, SP&S, and Chicago, Burlington & Quincy to form Burlington Northern, marking the end of an era.

Nevertheless, the legacy of the GN endures, remembered for its foundational role in fostering economic growth and connecting the American continent. Its routes continue to be an essential part of North America's rail network today, operated by BNSF Railway, a testament to Hill's enduring vision and the railway's historical significance.

The 4-6-6-4s

Northern Pacific's route to the Pacific Northwest was longer and featured more gradients than its competitor, the Great Northern. However, as the first transcontinental railroad to the Puget Sound, NP was a true trailblazer, forging a route where none had previously been.

Much like Union Pacific, NP needed swift and cutting-edge locomotives to maintain its edge. Collaborating with Alco, NP designed a 4-6-6-4 suited to its requirements, and in 1936, they received twelve of these powerful machines.

These "Challengers" were larger than UP's. Due to burning low-grade lineside coal, which had just two-thirds the heat of premium bituminous coal, NP's 4-6-6-4s were built with gigantic fireboxes, boasting a grate area of 152 square feet.

These engines made their mark across the expansive terrains and steep mountain paths of Montana and North Dakota, primarily hauling freight. However, during the late 1930s, the "Challengers" also powered the prestigious North Coast Limited between Missoula and Livingston, Montana.

NP eventually ordered 35 more 4-6-6-4s, organized into three separate classes, marking the last steam engines the railroad acquired. The Spokane, Portland & Seattle received eight oil-burning models of the NP type as well.

Data Sheet

Attribute	Z-6 (Locobase 7574)	Z-8 (Locobase 7575)
Class	Z-6	Z-8
Locobase ID	7574	7575
Railroad	Spokane, Portland & Seattle
Country	USA
Whyte	4-6-6-4
Number in Class	6	2
Road Numbers	900-905	910-911
Gauge	Std
Number Built	6	2
Builder	Alco-Schenectady
Year	1937	1944
Valve Gear	Walschaert
Driver Wheelbase (ft / m)	24.34 / 7.42	24.67 / 7.52
Engine Wheelbase (ft / m)	61.83 / 18.85	62.17 / 18.95
Ratio of Driving Wheelbase to Overall Engine Wheelbase	0.39	0.40
Overall Wheelbase (engine & tender) (ft / m)	113.67 / 34.65	110.04 / 33.54
Axle Loading (lbs / kg)	73,400 / 33,294	74,000 / 33,566
Weight on Drivers (lbs / kg)	436,000 / 197,767	444,000 / 201,395
Engine Weight (lbs / kg)	621,000 / 281,681	644,000 / 292,114
Tender Loaded Weight (lbs / kg)	410,000 / 185,973	437,000 / 198,220
Total Engine and Tender Weight (lbs / kg)	1,031,000 / 467,654	1,081,000 / 490,334
Tender Water Capacity (gals / ML)	20,000 / 75.76	25,000 / 94.70
Tender Fuel Capacity (gals / Liters)	6000 / 22,710	6500 / 24,603
Minimum Weight of Rail (lb/yd / kg/m)	121 / 60.50	123 / 61.50
Driver Diameter (in / mm)	69 / 1753	70 / 1778
Boiler Pressure (psi / kPa)	250 / 1720	260 / 1790
High Pressure Cylinders (dia x stroke) (in / mm)	23" x 32" / 584 x 813 (4)	23" x 32" / 584 x 813 (4)
Tractive Effort (lbs / kg)	104,267 / 47,294.77	106,888 / 48,483.64
Factor of Adhesion	4.18	4.15

The SP&S, co-owned by GN and NP, primarily obtained its steam locomotives second-hand from its parent companies. Out of the 121 steam engines it possessed during its operation, only 30 were acquired new, and none were uniquely designed by the SP&S. Among its new acquisitions were eight "Challengers" - which were essentially add-ons to NP's orders - during a 1937 power shortage. In 1944, two more "Challengers" joined the fleet via another NP order.

The 1937 addition, built by the American Locomotive Company, was akin to NP's Class Z-6 engines but differed by using oil as fuel instead of coal.

Alco classified as Class Z-6 for the SP&S, these locomotives bore road numbers 900 to 905. They featured 23 x 32 cylinders, 69-inch drivers, and a 250 psi boiler pressure, generating 104,500 pounds of tractive effort and weighing in at 621,000 pounds.

Engines 900 and 901 were quickly sold to Great Northern, only to return later. Number 900, which became GN #4000, came back in 1946, and number 901, GN #4001, returned in 1950 after being deemed superfluous.

As for the two additional 1944 Alco-built 4-6-6-4s, they also operated on oil and were classified as Class Z-8, receiving road numbers 910 and 911.

They came with 23 x 32 cylinders, 70-inch drivers, 260 psi boiler pressure, offered a tractive effort of 107,000 pounds, and tipped the scales at 639,000 pounds. Ultimately, the SP&S's fleet of eight "Challengers" was retired and scrapped by 1960.