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: