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.

Union Pacific Challenger
Union Pacific 3985 running through Alton, Iowa on October 1, 2008
Type and origin
Power typeSteam
BuilderAmerican Locomotive Company (ALCO)
Build date1936–1944
Total produced105 + (6 Denver & Rio Grande Western)
  UIC(2′C)C2′ h4
Gauge4 ft 8 12 in (1,435 mm) standard gauge
Driver dia.69 in (1,753 mm)
Wheelbase60 ft 4 12 in (18.402 m) Engine
121 ft 10 78 in (37.157 m) Engine + tender
Adhesive weight404,000 lb (183,251 kg)
Loco weight627,900 lb (284,800 kg)
Tender weight446,000 lb (202,000 kg)
Total weight1,073,900 lb (487,100 kg)
Fuel typeCoal, fuel oil
Fuel capacity32 short tons (29 t; 29 long tons)
6,450 US gal (24,400 l; 5,370 imp gal) UP3985
Water cap25,000 US gal (95,000 l; 21,000 imp gal)
  Firegrate area
132 sq ft (12 m2) (grate removed in 1990)
Boiler94 in (2,400 mm)
Boiler pressure280 lbf/in2 (1.93 MPa)
Heating surface4,795 sq ft (445.5 m2)
  Tubes527 sq ft (49.0 m2)
  Flues3,687 sq ft (342.5 m2)
  Firebox602 sq ft (55.9 m2)
  Heating area2,162 sq ft (200.9 m2)
Cylinder size21 in × 32 in (533 mm × 813 mm)
Performance figures
Maximum speed70 mph (110 km/h)
Tractive effort97,350 lbf (433.03 kN)
OperatorsUnion Pacific Railroad
ClassCSA-1, CSA-2, 4664-3, 4664-4, 4664-5
Preserved3977, 3985
DispositionTwo preserved, remainder scrapped

A total of 105 Challengers were built in five classes. They were nearly 122 ft (37 m) long and weighed 537 short tons (487 tonnes). 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, with the most notable example being Union Pacific 3985, the second-largest operable steam locomotive in the world, though it has been out of sevice since 2010.



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 280 lbf/in2 (1.93 MPa) boiler pressure, something usually reserved for passenger locomotives like the FEF Series. They had 69-inch (1,800 mm) 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 60 miles per hour (100 km/h), 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 3,600-short-ton (3,300 t; 3,200-long-ton) 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 317 short tons (288 t; 283 long tons) accompanied by a tender weighing 174 short tons (158 t; 155 long tons) when 2/3 loaded. Calculated tractive effort is 97,350 lbf (433.0 kN). 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.


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.

Table of orders and numbers[1]
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
1943 3975–3999 31 built but only 25 delivered to UP (see below); 3975–3984 converted to oil fuel in 1945; renumbered 3708–3717 in 1952. No. 3985 in excursion service from 1981 to 2010. No. 3977 on display at Cody Park.
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 & 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.[2] These were later sold to Clinchfield Railroad in 1947 and were renumbered as 670-675, where they formed the Clinchfield's Class E-3.[3]


Only two of the original 105 Challengers survive today, both which are from the 4664-4 order built in 1943. One remains on static display, while the other has been restored to operating condition by Union Pacific as part of its steam program.

Surviving Challenger locomotives
Type Number Image Date built Serial number Location Coordinates Notes
4664-4 3977 June 1943 70160 Cody Park, North Platte, Nebraska 41.147853°N 100.753113°W / 41.147853; -100.753113 (Challenger 3977) Displayed next to EMD DDA40X #6922.
4664-4 3985 July 1943 70174 Union Pacific Railroad, Cheyenne, Wyoming 41°7′46.9308″N 104°48′49.1688″W No. 3985 was restored in 1981 and used by Union Pacific on excursions until October 14, 2010, when mechanical problems led it to be taken out of service.

See also


  1. Drury 2015, p. 319.
  2. Kalmbach, A.C., ed. (August 1944). "Almost Identical Twins". Trains Magazine. 4: 29.
  3. Solomon 2009, p. 72.


  • Drury, George (2015). Guide to North American Steam Locomotives (2nd ed.). Kalmbach Publishing. ISBN 978-1-62700-259-2.
  • Solomon, Brian (2009). Alco Locomotives (1st ed.). Voyageur Press. ISBN 978-0-7603-3338-9.

Further reading

  • Ehernberger, James L. (1993). Union Pacific Steam Challenger Portraits (1st ed.). Challenger Press. ASIN B000TXFDIC.
  • Kratville, William W. (1980). The Challenger Locomotives (1st ed.). Kratville Publications. ASIN B0006E9WN6.
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