# 4-6-6-4

In the Whyte notation for classifying steam locomotives by wheel arrangement, a 4-6-6-4 is a railroad steam locomotive that has four leading wheels followed by two sets of six coupled driving wheels and four trailing wheels. 4-6-6-4's are commonly known as Challengers.
A similar wheel arrangement exists for Garratt locomotives, on which both engine units swivel, but is referred to as 4-6-0+0-6-4.
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 simple articulated 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 subsequently donated to the Railroading Heritage of Midwest America, where it is now undergoing a second restoration. The second example, Union Pacific 3977 is on static display in Cody Park North Platte, Nebraska.
Though originally intended for freight service, many units could be found for 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 | Scrapped between 1953 and 1958 |
| Clinchfield Railroad (12 new, 6 secondhand) | E-2 | 660–663 | ALCO | 1947 | Scrapped between 1955 and 1959 |
| Clinchfield Railroad (12 new, 6 secondhand) | 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 1951 and 1956 |
| Denver and Rio Grande Western Railroad (21) | L-105 | 3710–3714 | Baldwin | 1942 | Scrapped between 1951 and 1956 |
| Denver and Rio Grande Western Railroad (21) | L-97 | 3800–3805 | ALCO | 1943 | Diverted to the D&RGW from a UP order. 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 |
| Northern Pacific Railway (47) | Z-7 | 5121–5126 | ALCO | 1941 | Scrapped between 1951 and 1954 |
| Northern Pacific Railway (47) | 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 |
| Spokane, Portland and Seattle Railway (8) | 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 |
| Union Pacific Railroad (105) | CSA-2 | 3915–3939 | ALCO | 1937 | To UP 3815–3839. Scrapped 1958 |
| Union Pacific Railroad (105) | 4664-3 | 3950–3969 | ALCO | 1942 | Scrapped between 1958 and 1959 |
| Union Pacific Railroad (105) | 4664-4 | 3975–3999 | ALCO | 1943 | No. 3977 and 3985 are preserved. |
| Union Pacific Railroad (105) | 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 |