2-6-6-4

In the Whyte notation for the classification of steam locomotive wheel arrangement, a 2-6-6-4 is a locomotive with a two-wheel leading truck, two sets of six driving wheels, and a four-wheel trailing truck.

The 2-6-6-4 was a fairly late development, a product of the superpower steam concept, introduced by the Lima Locomotive Works, which encouraged the use of large fireboxes supported by four-wheel trailing trucks.

Such a firebox could sustain a rate of steam generation to meet any demands of the locomotive's cylinders, even at high speed.

High speeds were certainly among the design goals for the 2-6-6-4; most of the type were intended for use on fast freight trains.

The first 2-6-6-4s built in the United States were for the Pittsburgh and West Virginia Railroad, and these were not high-speed locomotives but rather mountain engines.