Union Pacific No. 3985 Challenger passing through Alton, IA in 2008.

(Mark Evans, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons)



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.

A similar wheel arrangement exists for Garratt locomotives, but is referred to as 4-6-0+0-6-4. On Garratt locomotives, both engine units pivot.

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.


The 4-6-6-4 Wheel Arrangement. Front of locomotive on left.

(Gwernol, Public domain, via Wikimedia Commons)


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 leading passenger consists as well. Railroads that used the Challenger type locomotive are shown below.


One-piece four-wheel Delta type trailer truck of Alco Challenger locomotive, 1936.

(Altona, CC0, via Wikimedia Commons)


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 1952 and 1958
Denver and Rio Grande Western Railroad (21) L-105 3710–3714 Baldwin 1942 Scrapped between 1954 and 1956
Denver and Rio Grande Western Railroad (21) 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
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

Advertising ink blotter depicting the Western Maryland Railway's new Baldwin 4-6-6-4 M2 locomotives.

(Western Maryland Railway, Public domain, via Wikimedia Commons)


Two WM M-2 4-6-6-4s with No. 1205 in the foreground.

(Western Maryland Railway, Public domain, via Wikimedia Commons)