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
Configuration 4-6-6-4
UIC classification (2′C)C2′ h4
Gauge 4 ft 8 12 in (1,435 mm)
Driver diameter 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
Weight on drivers 404,000 lb (183.3 t)
Locomotive weight 627,900 lb (284,800 kg)
Tender weight 446,000 lb (202,000 kg)
Locomotive and tender
combined 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)
6,450 US gal (24,400 l; 5,370 imp gal) UP3985
Water capacity 25,000 US gal (95,000 l; 21,000 imp gal)
Boiler pressure 280 lbf/in2 (1.93 MPa)
Heating surface:
– Tubes
527 sq ft (49.0 m2)
– Flues 3,687 sq ft (342.5 m2)
– Firebox 500 sq ft (46 m2)
– Total 4,795 sq ft (445.5 m2)
Superheater area 2,162 sq ft (200.9 m2)
Cylinders Four
Cylinder size 21 in × 32 in (533 mm × 813 mm)
Performance figures
Maximum speed 70 mph (110 km/h)
Tractive effort 97,350 lbf (433.03 kN)
Operator(s) Union Pacific Railroad
Class CSA-1, CSA-2, 4664-3, 4664-4, 4664-5

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 feet long and weighed more than one million pounds[clarification needed]. 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 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
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, used for excursion services by Union Pacific and Union Pacific 3977, on static display in North Platte, NE, at 41°08′52″N 100°45′11″W / 41.1478°N 100.752945°W / 41.1478; -100.752945.

External links[edit]


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