4-6-6-4

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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.

The equivalent UIC classification is (2'C)C2'.

This wheel arrangement was used by the Challenger class of locomotives on the Union Pacific Railroad. One locomotive of this class, 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.

Though originally intended for freight service, many units could be found leading passenger consists as well. Railroads that used the Challenger type locomotive include:

Railroad	Quantity, Builder
Carolina, Clinchfield & Ohio	12 ALCO, 6 from D&RGW
Delaware and Hudson	40 ALCO
Denver and Rio Grande Western	6 ALCO, 15 Baldwin
Great Northern	2 from SP&S
Northern Pacific	47 ALCO
Spokane, Portland & Seattle	8 ALCO
Union Pacific	105 ALCO
Western Maryland	12 Baldwin
Western Pacific	7 ALCO