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 (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.
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 (quantity)||Class||Road numbers||Builder||Build year||Notes|
|Clinchfield Railroad (12 new, 6 secondhand)||E-1||650–657||ALCO||1942–1943||Scrapped between 1953 and 1958|
|E-2||660–663||ALCO||1947||Scrapped between 1955 and 1959|
|E-3||670–675||ALCO||1943||Ex-D&RGW, acquired 1947 . Scrapped 1959|
|Delaware and Hudson Railway (40)||J||1500–1539||ALCO||1940–1946||Scrapped between 1951 and 1959|
|Denver and Rio Grande Western Railroad (21)||L-105||3700–3709||Baldwin||1938||Scrapped between 1952 and 1958|
|L-105||3710–3714||Baldwin||1942||Scrapped between 1954 and 1956|
|L-97||3800–3805||ALCO||1943||To Clinchfield Railroad in 1947 . Scrapped 1959|
|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 . Scrapped between 1953 and 1957|
|Northern Pacific Railway (47)||Z-6||5100–5120||ALCO||1936–1937||Scrapped between 1950 and 1953|
|Z-7||5121–5126||ALCO||1941||Scrapped between 1951 and 1954|
|Z-8||5130–5149||ALCO||1943–1944||Scrapped between 1952 and 1957|
|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 . Scrapped between 1957 and 1958|
|CSA-2||3915–3939||ALCO||1937||To UP 3815–3839 . Scrapped 1958|
|4664-3||3950–3969||ALCO||1942||Scrapped between 1958 and 1959|
|4664-4||3975–3999||ALCO||1943||No. 3977 and 3985 are preserved.|
|4664-5||3930–3949||ALCO||1944||Scrapped between 1957 and 1959|
|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. Scrapped 1958|
|Western Pacific Railroad (7)||M-100||401–407||ALCO||1938||Scrapped between 1953 and 1959|
- "Union Pacific 4-6-6-4 "Challenger" Locomotives in the USA". www.steamlocomotive.com. Retrieved 2019-05-08.
- Keefe, Kevin. "The Challenger at high tide". Classic Trains.
- The Challenger Type Locomotive
- The Union Pacific Challenger Roster
- Web Site of ToyTrains1 4-6-6-4 Challenger Steam Locomotives