Union Pacific Challenger

The Union Pacific Challengers are a type of simple articulated 4-6-6-4 steam locomotive built by American Locomotive Company (ALCO) from 1936 to 1944 and operated by the Union Pacific Railroad until the late 1950s.

Union Pacific Challenger
UP 3985 Westbound at Sloat, CA.jpg
Union Pacific 3985 running through Sloat, California on September 2, 2005
Type and origin
Power typeSteam
BuilderAmerican Locomotive Company (ALCO)
Build date1936–1944
Total produced105 + (6 Denver & Rio Grande Western)
 • Whyte4-6-6-4
 • 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, 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
PreservedTwo (Nos. 3985 and 3977)
RestoredNo. 3985; April 1981
DispositionNo. 3985 in storage, No. 3977 on display, 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 shaped the design of the last three orders of Challengers.

Two Union Pacific Challengers survive. The most notable is Union Pacific No. 3985, which was restored by the Union Pacific in 1981, then operated in excursion service as part of its heritage fleet program. Mechanical problems forced it out of service in October 2010; it was retired in January 2020 after the restoration of the 4-8-8-4 Big Boy 4014.[1][2] The only other surviving Challenger is UP No. 3977, which is on display in North Platte, Nebraska.



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. Previously, 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.[clarification needed] 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 Boy, 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 grades across Wyoming; the 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[3]
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. All Scrapped between 1957 and 1958
CSA-2 25 American Locomotive Company 68924–68948 1937 3915–3939 Converted to oil fuel; renumbered 3815–3839 in 1944. None preserved. All Scrapped 1958.
4664-3 20 American Locomotive Company 69760–69779 1942 3950–3969 3968 converted to oil fuel in 1946, renumbered 3944 in 1946. None preserved. All Scrapped between 1958 and 1959.
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. Retired in January 2020. 3977 preserved and on static display. Remainder scrapped in 1957.
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. All Scrapped between 1957 and 1959.

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.[4] These were later sold to Clinchfield Railroad in 1947 and were renumbered as 670-675, where they formed the Clinchfield's Class E-3.[5]


Only two Challengers survive, both from the 4664-4 order built in 1943. No. 3977 is on static display in North Platte, Nebraska. No. 3985 was restored to operating condition by Union Pacific in 1981 and used in excursion service until mechanical problems sent it back into storage in October 2010. It was officially retired in January 2020 and is stored in Cheyenne, Wyoming.

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°08′52″N 100°45′11″W / 41.147853°N 100.753113°W / 41.147853; -100.753113 (Challenger 3977) Displayed next to EMD DDA40X #6922. Painted in two-tone gray passenger scheme.
4664-4 3985   July 1943 70174 Union Pacific Railroad, Cheyenne, Wyoming 41°7′46.9308″N 104°48′49.1688″W / 41.129703000°N 104.813658000°W / 41.129703000; -104.813658000 (Challenger 3985) No. 3985 was restored in 1981 and used by Union Pacific on excursions until October 14, 2010, when mechanical problems forced it from service. It was officially retired in January 2020. The locomotive's tender, No. 25-C-311, has since April 2019 been assigned to the restored Big Boy, No. 4014.

See alsoEdit


  1. ^ Keefe, Kevin. "The Challenger at high tide". Classic Trains.
  2. ^ "Union Pacific No. 3985's next stop - Trains Magazine - Trains News Wire, Railroad News, Railroad Industry News, Web Cams, and Forms". cs.trains.com. Retrieved 2020-04-05.
  3. ^ Drury 2015, p. 319.
  4. ^ Kalmbach, A.C., ed. (August 1944). "Almost Identical Twins". Trains Magazine. 4: 29.
  5. ^ 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 readingEdit

  • 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.

External linksEdit