HO Blue Line 2-6-6-4 Class A w/Sound, N&W #1235
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. All 2-6-6-4s have been articulated locomotives, of the Mallet or related simple articulated type.
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 a 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 & West Virginia Railroad, and these were not high speed locomotives but rather heavy mountain luggers. They received three in 1934 and four more in 1937.
The final class of 2-6-6-4s was the Norfolk & Western Railway's Class A, built starting in 1936. 43 were built until 1950. They were the largest and most powerful 2-6-6-4s, capable of 6,300 drawbar horsepower (4.7 MW) at 45 mph (72 km/h) and able to run at over 70 mph (110 km/h), but were also capable of lugging heavy unit coal trains. They were used until dieselisation in 1959. One locomotive, Norfolk & Western 1218, was preserved and in 1987 was restored to running order, running on frequent excursions until 1991. Today it is on display at the Virginia Museum of Transportation.