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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
Railroad (quantity) Class Road numbers Builder Build year Notes
Clinchfield Railroad (12 new, 6 secondhand) E-1 650–657 ALCO 1942–1943
E-2 660–663 ALCO 1947
E-3 670–675 ALCO 1943 Ex-D&RGW, acquired 1947
Delaware and Hudson Railway (40) J 1500–1539 ALCO 1940–1946
Denver and Rio Grande Western Railroad (21) L-105 3700–3709 Baldwin 1938
L-105 3710–3714 Baldwin 1942
L-97 3800–3805 ALCO 1943 To Clinchfield Railroad in 1947
Great Northern Railway (2 secondhand) Z-6 4000–4001 ALCO 1937 Ex-SP&S 903–904; sold back to SP&S March 1950 and July 1946 respectively
Northern Pacific Railway (47) Z-6 5100–5120 ALCO 1936–1937
Z-7 5121–5126 ALCO 1941
Z-8 5130–5149 ALCO 1943–1944
Spokane, Portland and Seattle Railway (8) Z-6 900–905 ALCO 1937 Identical to NP Z-6 class; 903–904 sold to GN January 1940; purchased back March 1950 and July 1946 respectively
Z-8 910–911 ALCO 1944 Identical to NP Z-8 class
Union Pacific Railroad (105) CSA-1 3900–3914 ALCO 1936 To UP 3800–3814
CSA-2 3915–3939 ALCO 1937 To UP 3815–3839
4664-3 3950–3969 ALCO 1942
4664-4 3975–3999 ALCO 1943
4664-5 3930–3949 ALCO 1944
Western Maryland Railway (12) M-2 1201–1212 Baldwin 1940–1941 These and the 15 units for the D&RGW were the only ones of this type made by Baldwin.
Western Pacific Railroad (7) M-100 401–407 ALCO 1938

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