Railroads & Locomotives Locomotives Steam locomotive profile: 4-6-6-4 Challenger

Steam locomotive profile: 4-6-6-4 Challenger

By Neil Carlson | June 13, 2006

| Last updated on November 3, 2020

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Better than most railroads, perhaps, the Union Pacific understood fast freight service. With an expansive network of lines spread across the western states, the railroad had to maintain fast schedules in order to remain competitive.

Mindful of this, UP purchased the first heavy fast freight locomotives: unique three-cylinder 4-12-2s, built by Alco from 1926 to 1930. By 1934 the railroad wanted to expand its fleet of fast freight engines, but desired an engine that could negotiate tighter curves than was permissible with the rigid wheelbase of the 4-12-2.

Union Pacific was one of the few railroads with the wherewithal to buy new power during the Great Depression, so it came as no surprise when, together with Alco, the railroad set about designing a new fast freight steam engine.

Alco took the lead in the design effort. Well aware of the instability problems of Baltimore & Ohio’s 2-6-6-2s, Alco elected to employ a four-wheel engine truck – creating essentially a 4-12-2 split in half.

The key element in addressing the instability problem was to move the boiler forward in order to shift more weight onto the front engine. This meant that the firebox could not be placed behind the rear set of driving wheels, as with a 2-6-6-4. Instead, the firebox was supported both by the rear pair of drivers and the trailing truck, resulting in a long and shallow design.

Hence, the new locomotive would be a 4-6-6-4.

Union Pacific 4-6-6-4 Challenger No. 3976
Union Pacific Challenger No. 3976 was one of six coal-burning 4-6-6-4s later converted to oil and assigned to passenger train service. In 1946, the engine received smoke deflectors and a two-tone gray paint scheme with yellow striping.
Union Pacific
Alco delivered thirteen engines in 1936. Union Pacific was not disappointed, and over the next seven years went on to order a total of 105 4-6-6-4s built to two different designs: the early Alco design and a later UP design. A Union Pacific official christened the engine “Challenger.” Six of the locomotives were fitted for passenger service.

While UP and Alco were designing the Challenger, the Northern Pacific began shopping for new fast freight power.

NP’s transcontinental route to the Pacific Northwest was longer than that of its rival, the Great Northern, and had more grades as well. Like UP, Northern Pacific needed fast, modern engines to stay competitive, and for the same reasons as UP, worked with Alco to design a 4-6-6-4 that could meet its needs. The railroad took delivery of twelve 4-6-6-4s from Alco in 1936.

Northern Pacific’s Challengers were bigger than Union Pacific’s. The NP burned low-quality lineside coal with only two-thirds the heat value of good bituminous coal. Therefore, its 4-6-6-4s had enormous fireboxes with a grate area of 152 square feet.

NP put its Challengers to work in the wide-open spaces and mountain grades of Montana and North Dakota where they quickly made a name for themselves. They hauled mostly freight, although in the late 1930s, Challengers were used to power the railroad’s crack passenger train, the North Coast Limited, between Missoula and Livingston, Mont.

The Northern Pacific went on to order 35 more 4-6-6-4s in three different classes-the last steam engines bought by the railroad. Subsidiary Spokane, Portland & Seattle also received eight oil-burning versions of the NP Challengers.

Alco promoted its 4-6-6-4 to other railroads. The Western Pacific bought seven coal-burning engines in 1938 for use in the desert between Salt Lake City, Utah, and Elko, Nev.

Delaware & Hudson 4-6-6-4 Challenger No. 1521
Delaware & Hudson 4-6-6-4 No. 1521 charges upgrade toward Ararat Summit in northeast Pennsylvania on June 5, 1948. The photographer notes that two more Challengers were used as helper power at the rear of the train.
Charles A. Elston
The Delaware & Hudson took delivery of 40 engines from 1940 to 1946. With coal traffic declining, D&H needed fast locomotives to expand its role as a bridge carrier linking Canada and New England with the eastern United States. The Challengers hauled trains over Richmondville Hill and Belden Hill from Mechanicville to Binghamton, N.Y., and over Ararat Summit to Wilkes-Barre, Pa.

The Clinchfield purchased fourteen engines in 1942 and 1943 to haul coal and manifest freight through the Appalachian Mountains in Tennessee and North Carolina.

Rio Grande 4-6-6-4 No. 3705
Rio Grande’s Baldwin-built 4-6-6-4s were the last steam locomotives purchased by the railroad. The “rising sun” striping, worn only by the Rio Grande’s Challengers, was added to improve grade crossing visibility. No. 3705 rests at Helper, Utah, on July 5, 1940.
J. W. Maxwell
Baldwin, too, built Challengers. Denver & Rio Grande Western took delivery of fifteen Baldwins between 1938 and 1942. They found a home operating between Minturn, Colo., and Salt Lake City, Utah. Baldwin also built twelve engines for the Western Maryland in 1940 and 1941, which were used in the Allegheny Mountains west of Cumberland, Md.

The final Challengers were four built for the Clinchfield in 1947. By this time a total of 252 had been delivered.

Two Union Pacific 4-6-6-4s were saved and one, No. 3985, operates today in excursion service as part of Union Pacific’s steam program.

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