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 | Class | Road numbers | Builder | Build year | Notes |
| Clinchfield Railroad | E-1 | 650–657 | ALCO | 1942–1943 | |
| Clinchfield Railroad | E-2 | 660–663 | ALCO | 1947 | |
| Clinchfield Railroad | E-3 | 670–675 | ALCO | 1943 | Ex-D&RGW, acquired 1947 |
| Delaware and Hudson Railway | J | 1500–1539 | ALCO | 1940–1946 | |
| Denver and Rio Grande Western Railroad | L-105 | 3700–3709 | Baldwin | 1938 | - |
| Denver and Rio Grande Western Railroad | L-105 | 3710–3714 | Baldwin | 1942 | |
| Denver and Rio Grande Western Railroad | L-97 | 3800–3805 | ALCO | 1943 | To Clinchfield Railroad in 1947 |
| Great Northern Railway | 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 | Z-6 | 5100–5120 | ALCO | 1936–1937 | |
| Northern Pacific Railway | Z-7 | 5121–5126 | ALCO | 1941 | |
| Northern Pacific Railway | Z-8 | 5130–5149 | ALCO | 1943–1944 | |
| Spokane, Portland and Seattle Railway | 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 |
| Spokane, Portland and Seattle Railway | Z-8 | 910–911 | ALCO | 1944 | Identical to NP Z-8 class |
| Union Pacific Railroad | CSA-1 | 3900–3914 | ALCO | 1936 | To UP 3800–3814 |
| Union Pacific Railroad | CSA-2 | 3915–3939 | ALCO | 1937 | To UP 3815–3839 |
| Union Pacific Railroad | 4664-3 | 3950–3969 | ALCO | 1942 | |
| Union Pacific Railroad | 4664-4 | 3975–3999 | ALCO | 1943 | No. 3977 and 3985 are preserved. |
| Union Pacific Railroad | 4664-5 | 3930–3949 | ALCO | 1944 | |
| Western Maryland Railway | 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 | M-100 | 401–407 | ALCO | 1938 |