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 (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:
Q1147963
Whyte notation
MISC
Q3565868
railroad
Concept
Q171043
steam locomotive
Concept
Q2043007
leading wheel
Concept
Q284786
driving wheel
Concept
Q1385403
trailing wheel
Concept
Q1142359
UIC classification
MISC
Q1142359
Italian classification
MISC
Q5502303
French classification
UNK
Q7855377
Turkish classification
UNK
Q376820
Swiss classification
MISC
Q1142359
UIC classification
MISC
Q725793
Union Pacific Railroad
ORG
Q15700457
Union Pacific Challenger
MISC
Q933916
Union Pacific Big Boy
MISC
Q4637095
4-8-8-4
MISC
Q425953
Union Pacific 3985
MISC
Q5419725
excursion service
Concept
Q15700457
Union Pacific 3977
MISC
Q989960
North Platte, Nebraska
LOC
Q464411
4-6-6-4
MISC
Q464411
4-6-6-4
MISC
Q464411
4-6-6-4
MISC
3985-01-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
3985
DATE
1981-01-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
1981
DATE
2010-01-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
2010
DATE
2020-01-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
January 2020
DATE
3977-01-01T00:00:00Z^^http://www.w3.org/2001/XMLSchema#dateTime
3977
DATE
+4^^http://www.w3.org/2001/XMLSchema#decimal
4
NUMBER
-6^^http://www.w3.org/2001/XMLSchema#decimal
-6
NUMBER
-6^^http://www.w3.org/2001/XMLSchema#decimal
-6
NUMBER
-4^^http://www.w3.org/2001/XMLSchema#decimal
-4
NUMBER
+4^^http://www.w3.org/2001/XMLSchema#decimal
4
NUMBER
-6^^http://www.w3.org/2001/XMLSchema#decimal
-6
NUMBER
-6^^http://www.w3.org/2001/XMLSchema#decimal
-6
NUMBER
-4^^http://www.w3.org/2001/XMLSchema#decimal
-4
NUMBER
+2^^http://www.w3.org/2001/XMLSchema#decimal
2
NUMBER
+2^^http://www.w3.org/2001/XMLSchema#decimal
2
NUMBER
++032^^http://www.w3.org/2001/XMLSchema#decimal
+032
NUMBER
+35^^http://www.w3.org/2001/XMLSchema#decimal
35
NUMBER
++35^^http://www.w3.org/2001/XMLSchema#decimal
+35
NUMBER
+3^^http://www.w3.org/2001/XMLSchema#decimal
3
NUMBER
+5^^http://www.w3.org/2001/XMLSchema#decimal
5
NUMBER
++3^^http://www.w3.org/2001/XMLSchema#decimal
+3
NUMBER
+5^^http://www.w3.org/2001/XMLSchema#decimal
5
NUMBER
+2^^http://www.w3.org/2001/XMLSchema#decimal
2
NUMBER
+2^^http://www.w3.org/2001/XMLSchema#decimal
2
NUMBER
+4^^http://www.w3.org/2001/XMLSchema#decimal
4
NUMBER
-8^^http://www.w3.org/2001/XMLSchema#decimal
-8
NUMBER
-8^^http://www.w3.org/2001/XMLSchema#decimal
-8
NUMBER
-4^^http://www.w3.org/2001/XMLSchema#decimal
-4
NUMBER
+4^^http://www.w3.org/2001/XMLSchema#decimal
4
NUMBER
-6^^http://www.w3.org/2001/XMLSchema#decimal
-6
NUMBER
-6^^http://www.w3.org/2001/XMLSchema#decimal
-6
NUMBER
-4^^http://www.w3.org/2001/XMLSchema#decimal
-4
NUMBER
part of
part of
part of
wheel arrangement
wheel arrangement