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. The UIC classification is refined to (2'C)C2' for Mallet locomotives. 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

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. The UIC classification is refined to (2'C)C2' for Mallet locomotives. Though originally intended for freight service, many units could be found leading passenger consists as well. Railroads that used the Challenger type locomotive include: