2-8-8-8-4

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Under the Whyte notation for the classification of steam locomotives, a 2-8-8-8-4 has two leading wheels, three sets of eight driving wheels, and a four trailing wheels. The equivalent UIC classification is (1'D)D(D2'). Because of its length, such a locomotive must be an articulated locomotive. It is not actually longer than a normal articulated; the third set of drivers is located under the tender. Only one was ever built, a Mallet-type for the Virginian Railway in the early 20th century. Like the same railroad's large articulated electrics and the Erie Railroad 2-8-8-8-2s, it was nicknamed "Triplex". The center set of drivers received high pressure steam. The exhaust from these was split between the two other sets of drivers. The front set exhausted through the smokebox and the rear set exhausted through a large pipe after heating the water in the tender. Since only half of the exhaust steam exited through the smokebox, firebox draft (and thus boiler heating) was poor. They all had undersized boilers, preventing them from running at high speed for very long.

With all six cylinders operating at high pressure (an unsustainable situation) the Triplexes produced more tractive effort than any other steam locomotive. The Triplexes could also be considered the largest tank locomotives ever built since the tender had driving wheels as well and thus contributed to traction.