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

In the 1930s, with freight traffic increasing, the Union Pacific Railroad had to use combinations of its 2-8-8-0 and 2-10-2 locomotives to get trains over the rugged grades of the Wahsatch Mountains. To stay competitive, a more powerful locomotive was needed to speed up the railroad and to reduce the rising cost of helpers and extra trains. The UP simply needed a locomotive that could climb the Wahsatch faster.

Arthur H. Fetter, the General Mechanical Engineer, had been designing locomotives for the Union Pacific since 1918, and had been responsible for the development of its 4-8-2 "Mountain" and 4-10-2 "Overland" locomotives as well as many other innovations and improvements to UP motive power. Fetter suggested a high speed articulated locomotive to reduce the reciprocating weight of a compound and to increase the 50 mph speed limit of the railroad's most powerful locomotives, the rigid wheeled 4-12-2s.

Fetter had a long standing working arrangement with the American Locomotive Company and he often collaborated with ALCO's engineers on locomotive designs. For the new more powerful locomotive he and the ALCO engineers started with the 4-12-2. They decided that the leading four wheel truck would be needed for better side control. They split the six sets of drivers into two groups of three and replaced the two 27" outside cylinders and the one 31" middle cylinder with four 22" x 32" cylinders. Two inches were added to the diameter of the boiler and the pressure was raised from 220 psi to 255 psi. The firebox was enlarged and they added a four wheel trailing truck to carry its added weight.

The first 4-6-6-4, UP number 3900, was received from ALCO at Council Bluffs on August 25, 1936, and after a brief ceremony it headed west pulling a refrigerator train.

During a meeting in 1936, in which Otto Jabelmann, the VP of Research, and William Jeffers, the Executive VP of the Union Pacific System listened to J. W. Burnett, the General Superintendent of Motive Power and Machinery, propose a test run for the new locomotive. Burnett had decided to operate it unassisted from Ogden to Wahsatch and then run fast over to Green River before turning back to Ogden with another train. Burnett said "that is a challenge for any locomotive" and Jeffers replied "it certainly is...let's call them "Challengers". After the meeting Jeffers sent a memo to the Advertising Department in which he said he wanted the name "Challenger" used in all press releases about the new locomotive.

The Union Pacific Railroad would buy a total of 105 "Challengers" and eight other railroads would use the other 147 of the total 252 that were built. The Baldwin Locomotive Works built 27 of the 4-6-6-4s. The American Locomotive Company built the rest.

Information for this introduction to Challengers provided by Richard Duley.

Builders of 4-6-6-4 "Challenger" Type Locomotives (by Richard Duley)

Railroad LineQuantity, Builder
Clinchfield12 ALCO, 6 from D&RGW
Delaware & Hudson40 ALCO
Denver & Rio Grande Western6 ALCO, 15 Baldwin
Great Northern2 from SP&S
Northern Pacific47 ALCO
Spokane, Portland & Seattle8 ALCO
Union Pacific105 ALCO
Western Maryland12 Baldwin
Western Pacific7 ALCO

Number of 4-6-6-4 "Challenger" Type Locomotives Built by Year (by Richard Duley)


Railroads that used 4-6-6-4 "Challenger" locomotives in the USA (data provided by Steve Llanso of Sweat House Media)

Surviving Examples of 4-6-6-4 "Challenger" Locomotives in the USA

No.ClassF.M. WhyteGaugeRailroad LineLocationStatusBuilder InfoNotes
3977466444-6-6-44'-8½"UP Cody Park, North Platte, NEdisplayALCO (Schenectady) #70160, 1943
3985466444-6-6-44'-8½"UP UP engine house, Cheyenne, WYout of serviceALCO (Schenectady) #70174, 1943Down for 1472 day inspection.

Web Pages


If you have any railroad data such as diagram books, rail station plans or anything else that you would be willing to share, please contact us.