4-6-6-4


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.
Other equivalent classifications are:

UIC classification: 2CC2

French classification: 230+032

Turkish classification: 35+35

Swiss classification: 3/5+3/5
The UIC classification is refined to C2' for Mallet locomotives.
Challengers were most common in the Union Pacific Railroad, but many other railroads ordered them as well. 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.
Today, the only Challenger locomotives that survive were both owned by Union Pacific. One such locomotive, Union Pacific 3985, was operated by the Union Pacific Railroad in excursion service from 1981 to 2010, when mechanical problems took it out of service. It was retired in January 2020 due to its poor mechanical condition, and currently remains stored in Cheyenne. The second example, Union Pacific 3977 is on static display in North Platte, Nebraska.
Though originally intended for freight service, many units could be found leading passenger consists as well. Railroads that used the Challenger type locomotive include:
Railroad ClassRoad numbersBuilderBuild yearNotes
Clinchfield Railroad E-1650–657ALCO1942–1943
Clinchfield Railroad E-2660–663ALCO1947
Clinchfield Railroad E-3670–675ALCO1943Ex-D&RGW, acquired 1947
Delaware and Hudson Railway J1500–1539ALCO1940–1946
Denver and Rio Grande Western Railroad L-1053700–3709Baldwin1938-
Denver and Rio Grande Western Railroad L-1053710–3714Baldwin1942
Denver and Rio Grande Western Railroad L-973800–3805ALCO1943To Clinchfield Railroad in 1947
Great Northern Railway Z-64000–4001ALCO1937Ex-SP&S 903–904; sold back to SP&S March 1950 and July 1946 respectively
Northern Pacific Railway Z-65100–5120ALCO1936–1937
Northern Pacific Railway Z-75121–5126ALCO1941
Northern Pacific Railway Z-85130–5149ALCO1943–1944
Spokane, Portland and Seattle Railway Z-6900–905ALCO1937Identical to NP Z-6 class; 903–904 sold to GN January 1940; purchased back March 1950 and July 1946 respectively
Spokane, Portland and Seattle Railway Z-8910–911ALCO1944Identical to NP Z-8 class
Union Pacific Railroad CSA-13900–3914ALCO1936To UP 3800–3814
Union Pacific Railroad CSA-23915–3939ALCO1937To UP 3815–3839
Union Pacific Railroad 4664-33950–3969ALCO1942
Union Pacific Railroad 4664-43975–3999ALCO1943No. 3977 and 3985 are preserved.
Union Pacific Railroad 4664-53930–3949ALCO1944
Western Maryland Railway M-21201–1212Baldwin1940–1941These and the 15 units for the D&RGW were the only ones of this type made by Baldwin.
Western Pacific Railroad M-100401–407ALCO1938