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Union Pacific Steam Locomotives

This page was last updated on September 3, 2010.

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The Union Pacific "Light" 4-6-6-4s

(Taken from Allen Copeland's original typewritten narrative dated October 15, 1978.)

With an increase in demand for fast freight service as the U.S. pulled out of the depression, the Union Pacific started searching once again for motive power improvements. The road liked their huge 4-12-2's, particularly their speed and power, but the long rigid wheelbase and maintenance costs of the inside (third) cylinder demanded improvement.

Fifteen 4-6-6-4's, numbers 3900-3914 were purchased from the American Locomotive Company in 1936. These locomotives were the first of their wheel arrangement and were named "Challengers" by the U.P. The first group achieved all that was expected of them and another batch was delivered from ALCO in 1937, numbers 3915-3939. The 4-6-6-4's were faster than the 4-12-2's and could be used all over the system.

All locomotives were delivered as coal burners, but 3934-3939 were converted to oil fuel soon after delivery for use on the Los Angeles & Salt Lake District. The engines were made so as to be convertable from one fuel to another, requiring only the installation of a seperate oil tank in the coal bunker and oil firing jets. Each set of these Challengers differed slightly in weight, but all were similar in appearance. With semi-Vanderbuilt tenders, Walschaerts valve gear, "Box-Pok" drivers and a centered headlight. The 4-6-6-4's had friction bearings on the drivers, but the lead, trailing and tender trucks had roller bearings.

The "Challengers" were considered dual service locomotives and were used in heavy passenger service as required. Early in 1943, 3900-3915 were changed to oil fuel, and in April 1944 the entire class was renumbered 3800-3839. In 1949, some of the oil burners were converted back to coal and given numbers in the 3700-series. In 1950, these coal burners were again converted to oil and reassigned their former 3800-series numbers. In 1948-49 ten of the 4-6-6-4's had the front engine frames replaced by new cast steel engine beds. Others had Commonwealth swing gate pilots, larger front sandboxes, air compressor shields and steps from the pilot up to the boiler.

The "light" Challengers (as they came to be known after heavier 4-6-6-4's were delivered in 1942-44) were initially assigned to Ogden-Green River and Ogden-Cheyenne freight service, but were soon reassigned to all parts of the far flung system, including the L.A. & S.L., O.S.L. and O.W.R.R.&N. in freight and passenger service. However, they rarely if ever ran on the Kansas Division. During the later days of steam operation, they were assigned to the Nebraska Division, running east from Cheyenne to Council Bluffs.