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. Challengers were most common in the Union Pacific Railroad and the Clinchfield Railroad.

This locomotive was impressive and widely used by the Union Pacific for passenger service but was mostly used for freight. 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.

One such locomotive, Union Pacific 3985 is still operable, and it is currently the largest operable steam locomotive in the world. Another example, Union Pacific 3977 is on static display in North Platte, Nebraska. 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.

Though originally intended for freight service, many units could be found 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
	E-2	660–663	ALCO	1947
	E-3	670–675	ALCO	1943	ex-D&RGW, acquired 1947
Delaware and Hudson Railway (40)	J	1500–1539	ALCO	1940–1946
Denver and Rio Grande Western Railroad (21)	L-105	3700–3709	Baldwin	1938
	L-105	3710–3714	Baldwin	1942
	L-97	3800–3805	ALCO	1943	to Clinchfield Railroad in 1947
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
Northern Pacific Railway (47)	Z-6	5100–5120	ALCO	1936–1937
	Z-7	5121–5126	ALCO	1941
	Z-8	5130–5149	ALCO	1943–1944
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
	CSA-2	3915–3939	ALCO	1937	to UP 3815–3839
	4664-3	3950–3969	ALCO	1942
	4664-4	3975–3999	ALCO	1943
	4664-5	3930–3949	ALCO	1944
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.
Western Pacific Railroad (7)	M-100	401–407	ALCO	1938

External links

• The Challenger Type Locomotive
• The Union Pacific Challenger Roster
• Web Site of ToyTrains1 4-6-6-4 Challenger Steam Locomotives