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 next of the type were a successful class of ten ordered by the Seaboard Air Line in 1935 and 1937. These were high speed freight engines and were very successful. Upon dieselisation , the road sold the locomotives to the Baltimore & Ohio Railroad in 1947, who operated them until 1953.

The final class of 2-8-8-4s 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). They were used until dieselisation in 1959. One locomotive, #1218, was preserved and in 1987 was restored to running order, running on frequent excursions until 1994. The Class A is believed by some to be the best class of steam locomotive produced in North America.

In all, 60 2-6-6-4s were constructed in North America.

References

• Trains.com article on the 2-6-6-4 type.