The Challenger Type Locomotive


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 Wahstach 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 Wahstach 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 Jablemann, the VP of Research, and William Jetters, 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.

Railroads that used Challengers

Railroad Line

Quantity, Builder


12 ALCO, 6 from D&RGW

Delaware & Hudson


Denver & Rio Grande Western

6 ALCO, 15 Baldwin

Great Northern

2 from SP&S

Northern Pacific


Spokane, Portland & Seattle


Union Pacific

105 ALCO

Western Maryland

12 Baldwin

Western Pacific



Surviving Challengers

Union Pacific 3977

This Challenger is on display in Codey Park, North Platte, NE. In 1952 it was converted to burn oil and renumbered 3710.

Union Pacific 3985

This is the Challenger that everyone knows about. The Union Pacific uses it on excursions. It had been on display in Cheyenne, WY for a number of years. In 1981 it was restored to operating condition by a group of volunteer Union Pacific employees. Today, its home is the UP engine house in Cheyenne, WY where it shares space with UP Northern 844 and a couple others.

Challenger References

First and Last Schedule
Gary McClure's Home Page I'VE BEEN EVERYWHERE Challenger Home Page