Union Pacific Challenger
The Union Pacific Challengers are a type of simple articulated 4-6-6-4 steam locomotive built by American Locomotive Company from 1936 to 1944 and operated by the Union Pacific Railroad until the late 1950s.
A total of 105 Challengers were built in five classes. They were nearly long and weighed 537 short tons. They operated over most of the Union Pacific system, primarily in freight service, but a few were assigned to the Portland Rose and other passenger trains. Their design and operating experience shaped the design of the Big Boy locomotive type, which in turn shapes the design of the last three orders of Challengers.
Today, only two Union Pacific Challengers survive. The most notable example being Union Pacific No. 3985, which was restored by the Union Pacific in 1981 as part of its heritage fleet program. However, due to mechanical problems, it was taken out of service in October 2010, and eventually retired in January 2020 from excursion service after the restoration of Big Boy 4014.
History
Description
The name "Challenger" was given to steam locomotives with a 4-6-6-4 wheel arrangement: four wheels in the leading pilot truck to guide the locomotive into curves, two sets of six driving wheels, and four trailing wheels to support the rear of the engine and its massive firebox. Each set of driving wheels is driven by two steam cylinders. In essence, the result is two engines under one boiler. Union Pacific developed five types of Challengers: the "light" CSA-1 and CSA-2 classes and the "heavy" 4664-3, 4664-4, and 4664-5 classes.The railroad sought powerful locomotives that could handle mountain grades at high speeds. Previous articulated locomotives had been limited to slow speeds by their design. Technical breakthroughs allowed the UP Challengers to operate with boiler pressure, something usually reserved for passenger locomotives like the FEF Series. They had drivers, mammoth wheels usually seen on passenger locomotives only, because freight engines normally require the extra torque provided by smaller wheels. Speeds in excess of, while unheard-of on other railroads using articulated steam locomotives, became commonplace on the Union Pacific.
When the first Challengers entered service in 1936, on the UP's main line over the Wasatch Range between Green River and Ogden, the locomotives had problems climbing the steep grades. For most of the route, the maximum grade is 0.82% in either direction, but the climb eastward from Ogden, into the Wasatch Range, reached 1.14%. Hauling a freight train demanded double heading and helper operations, and adding and removing helper engines slowed operations. Those limitations prompted the introduction of the Big Boy in 1941, as well as a redesign of the last three orders from 1942 to 1944.
Using the experience from the Big Boys, UP chief mechanical engineer, Otto Jabelmann, redesigned the last three orders of Challengers in 1941. The result was a locomotive in working order weighing some accompanied by a tender weighing when 2/3 loaded. Calculated tractive effort is. From 1941, the Challengers were intended to speed up freight operations on the 0.82% grades across Wyoming; the 1.14% Wasatch Range climb east from Ogden was to be conquered by the Big Boys without helpers.
Construction
The 105 locomotives were divided into five orders, which can be put into two groups: the first two orders of light Challengers, and the final three of heavy Challengers. Along with the Big Boys, the Challengers arrived on the scene just as traffic was surging in preparation for American participation in World War II.| Class | Quantity | Manufacturer | Serial Nos. | Year built | UP No. | Notes |
| CSA-1 | 15 | American Locomotive Company | 68745–68759 | 1936 | 3900–3914 | Converted to oil fuel in 1941–43; renumbered 3800–3814 in 1944. None preserved. |
| CSA-2 | 25 | American Locomotive Company | 68924–68948 | 1937 | 3915–3939 | Converted to oil fuel; renumbered 3815–3839 in 1944. None preserved. |
| 4664-3 | 20 | American Locomotive Company | 69760–69779 | 1942 | 3950–3969 | 3968 converted to oil fuel in 1946, renumbered 3944 in 1946. None preserved. |
| 4664-4 | 31 | American Locomotive Company | 70158–70162 70169–70182 70678–70683 | 1943 | 3975–3999 | 31 built but only 25 delivered to UP ; 3975–3984 converted to oil fuel in 1945; renumbered 3708–3717 in 1952. No. 3985 in excursion service from 1981 to 2010. |
| 4664-5 | 20 | American Locomotive Company | 72792–72811 | 1944 | 3930–3949 | 3930/31/32/34/37/38/43/44 converted to oil fuel in 1952 and renumbered 3700–3707. None preserved. |
As part of Union Pacific's fourth order in 1943, ALCO built thirty-one locomotives for Union Pacific using the same specifications. However, the War Production Board diverted six locomotives after completion to the Denver and Rio Grande Western Railroad via a lease through the War Department's Defense Plant Corporation. Locomotives 3900-3905 formed the Rio Grande's Class L-97. These were later sold to Clinchfield Railroad in 1947 and were renumbered as 670-675, where they formed the Clinchfield's Class E-3.