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A 4-6-6-4 Challenger Slideshow.

Diagram of the 4-6-6-4 Wheel Arrangement.

Snake River Farms Wagyu Brisket

Delaware & Hudson ALCO Class J No. 1505.

ALCO logo.

Union Pacific 4-6-6-4 Challenger No. 3985 Westbound at Sloat, California, Sept 2nd, 2005. Photo by Drew Jacksich.

By Drew Jacksich from San Jose, CA, California Republic [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

Union Pacific 4-6-6-4 Challenger No. 3985 Westbound at Sloat, California, Sept 2nd, 2005. Photo by Drew Jacksich.
Diagram of the 4-6-6-4 Wheel Arrangement.
Baldwin logo.

4-6-6-4 "Challenger" Steam Locomotive
In Whyte notation, a 4-6-6-4 is a railroad steam locomotive that has four leading wheels followed by six coupled driving wheels, a second set of six driving wheels and four trailing wheels. 4-6-6-4's are commonly known as Challengers. Challengers were most common in the Union Pacific Railroad and the Clinchfield Railroad.

This locomotive was impressive and widely used by the Union Pacific for passenger service but was mostly used for freight.

Other equivalent classifications are:
UIC classification: 2CC2 (also known as German classification and Italian classification)
French classification: 230+032
Turkish classification: 35+35
Swiss classification: 3/5+3/5

The UIC classification is refined to (2'C)C2' for Mallet locomotives.

One such locomotive, Union Pacific 3985 is still operable, and it is currently the largest operable steam locomotive in the world. Another example, Union Pacific 3977 is on static display in North Platte, Nebraska. An expansion for the Union Pacific Challenger class was the Union Pacific Big Boy class, being a 4-8-8-4, instead of a 4-6-6-4.

Though originally intended for freight service, many units could be found leading passenger consists as well. Railroads that used the Challenger type locomotive include:

4-6-6-4 Construction Roster
Clinchfield Railroad

Quantity: 12 new, 6 secondhand
Class: E-1
Road numbers: 650–657
Builder: ALCO
Build year: 1942–1943
Class: E-2
Road numbers: 660–663
Builder: ALCO
Build year: 1947
Class: E-3
Road numbers: 670–675
Builder: ALCO
Build year: 1943
Notes: ex-D&RGW, acquired 1947.

Delaware and Hudson Railway

Quantity: 40
Class: J
Road numbers: 1500–1539
Builder: ALCO
Build year: 1940–1946

Denver and Rio Grande Western Railroad

Quantity: 21
Class: L-105
Road numbers: 3700–3709
Builder: Baldwin
Build year: 1938
Class: L-105
Road numbers: 3710–3714
Builder: Baldwin
Build year: 1942
Class: L-97
Road numbers: 3800–3805
Builder: ALCO
Build year: 1943
Notes: to Clinchfield Railroad in 1947.

Great Northern Railway

Quantity: 2 secondhand
Class: Z-6
Road numbers: 4000–4001
Builder: ALCO
Build year: 1937
Notes: ex-SP&S 903–904; sold back to SP&S March 1950 and July 1946 respectively.

Northern Pacific Railway

Quantity: 47
Class: Z-6
Road numbers: 5100–5120
Builder: ALCO
Build year: 1936–1937
Class: Z-7
Road numbers: 5121–5126
Builder: ALCO
Build year: 1941
Class: Z-8
Road numbers: 5130–5149
Builder: ALCO
Build year: 1943–1944

Spokane, Portland and Seattle Railway

Quantity: 8
Class: Z-6
Road numbers: 900–905
Builder: ALCO
Build year: 1937
Notes: Identical to NP Z-6 class; 903–904 sold to GN January 1940; purchased back March 1950 and July 1946 respectively.
Class: Z-8
Road numbers: 910–911
Builder: ALCO
Build year: 1944
Notes: Identical to NP Z-8 class.

Union Pacific Railroad

Quantity: 105
Class: CSA-1
Road numbers: 3900–3914
Builder: ALCO
Build year: 1936
Notes: to UP 3800–3814.
Class: CSA-2
Road numbers: 3915–3939
Builder: ALCO
Build year: 1937
Notes: to UP 3815–3839.
Class: 4664-3
Road numbers: 3950–3969
Builder: ALCO
Build year: 1942
Class: 4664-4
Road numbers: 3975–3999
Builder: ALCO
Build year: 1943
Class: 4664-5
Road numbers: 3930–3949
Builder: ALCO
Build year: 1944

Western Maryland Railway

Quantity: 12
Class: M-2
Road numbers: 1201–1212
Builder: Baldwin
Build year: 1940–1941
Notes: These and the 15 units for the D&RGW were the only ones of this type made by Baldwin.

Western Pacific Railroad

Quantity: 7
Class: M-100
Road numbers: 401–407
Builder: ALCO
Build year: 1938

See also:

4-8-8-4

Steam Locomotives

American Locomotive Company

Baldwin Locomotive Works

Delaware & Hudson Class J No. 1505.
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