Union Pacific Challenger

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Union Pacific Challenger
Union Pacific Challenger 3985 01.jpg
UP 3985 running through Alton, Iowa in October 2008
Type and origin
Power type Steam
Builder American Locomotive Company (ALCO)
Build date 1936–1944
Total produced 105
Configuration 4-6-6-4
UIC class (2′C)C2′ h4
Gauge 4 ft 8 12 in (1,435 mm) standard gauge
Driver dia. 69 in (1,753 mm)
Wheelbase 60 ft 4 12 in (18.402 m) Engine
121 ft 10 78 in (37.157 m) Engine + tender
Adhesive weight 404,000 lb (183,251 kg)
Loco weight 627,900 lb (284,800 kg)
Tender weight 446,000 lb (202,000 kg)
Total weight 1,073,900 lb (487,100 kg)
Fuel type Coal, UP 3985 converted to No. 5 fuel oil
Fuel capacity 32 short tons (29 t; 29 long tons)
6,450 US gal (24,400 l; 5,370 imp gal) UP3985
Water cap 25,000 US gal (95,000 l; 21,000 imp gal)
Boiler pressure 280 lbf/in2 (1.93 MPa)
Heating surface 4,795 sq ft (445.5 m2)
 • Tubes 527 sq ft (49.0 m2)
 • Flues 3,687 sq ft (342.5 m2)
 • Firebox 500 sq ft (46 m2)
 • Heating area 2,162 sq ft (200.9 m2)
Cylinders Four
Cylinder size 21 in × 32 in (533 mm × 813 mm)
Performance figures
Maximum speed 60 mph (97 km/h)
Tractive effort 97,350 lbf (433.03 kN)
Operators Union Pacific Railroad
Class CSA-1, CSA-2, 4664-3, 4664-4, 4664-5
Preserved 2

The Union Pacific Challengers were a type of simple articulated 4-6-6-4 steam locomotive built by American Locomotive Company for the Union Pacific Railroad. 105 of these locomotives were built between 1936 and 1943. The Challengers were nearly 122 ft (37 m) long and weighed 314 tons (284,800 kg). They operated over most of the Union Pacific system, primarily in freight service, but a few were assigned to passenger trains operating through mountain territory to California and Oregon. The locomotives were built specifically for Union Pacific and much of the experience gained later went into the design of the "Big Boy".

The name "Challenger" was given to steam locomotives with a 4-6-6-4 wheel arrangement. This means that they have four wheels in the leading pilot truck, which helps guide the locomotive into curves, two sets of six driving wheels, and finally four trailing wheels, which support the rear of the engine and its massive firebox. Each set of six driving wheels is driven by two steam cylinders. In essence, the result is two engines under one boiler. The Union Pacific Railroad sponsored development of this type to meet the need for higher speeds in main-line service. Historically, articulated locomotives had been limited to slow speeds by factors inherent in their design. The technical breakthroughs achieved with the Challenger enabled the carrier to develop the Big Boy with the same speed expectations. Speeds in excess of 60 M.P.H., while unheard-of on other railroads using articulated steam locomotives, became commonplace on the Union Pacific.


The 105 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.

Table of orders and numbers
Class Quantity Manufacturer Serial Nos. Year 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
CSA-2 25 American Locomotive Company 68924–68948 1937 3915–3939 Converted to oil fuel; renumbered 3815–3839 in 1944
4664-3 20 American Locomotive Company 69760–69779 1942 3950–3969
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. 3985 in excursion service since 1981.
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.

As part of Union Pacific's fourth order in 1943, ALCO built 31 locomotives for Union Pacific using the same specifications. However, the War Production Board diverted 6 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.[1] These were later sold to Clinchfield Railroad in 1947, becoming 670-675.


Two examples survive today: Union Pacific 3985, which remains in operating condition and is used for excursion services by Union Pacific, and Union Pacific 3977, which is on static display on Cody Park at North Platte, NE. None of the original Challengers that were built for Union Pacific were preserved.

As of November 2016, 3985 is down indefinitely for repairs, placed second in line behind Union Pacific 4014 in the restoration order. 4014 is scheduled to be completed in 2019. Once 4014 is restored, crews will start work on 3985 for a 2-3 year overhaul. 3985 could be back in steam by 2021 (2022 at the latest).[citation needed]


  1. ^ Kalmbach, A.C., ed. (August 1944). "Almost Identical Twins". Trains Magazine. 4: 29. 

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