From TrainSpottingWorld, for Rail fans everywhere
The Matt H. Shay in a builder's photo

Under the Whyte notation for the classification of steam locomotives, a 2-8-8-8-2 has two leading wheels, three sets of eight driving wheels, and two trailing wheels. The equivalent UIC classification is (1'D)D(D1').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. All of the examples produced were of the Mallet type.

The Erie Railroad built the only three examples of the type in the early 20th century. The first was named Matt H. Shay; all three, as well as the lone 2-8-8-8-4 and several Virginian Railway electrics, shared the nickname "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 first through a feedwater heater in the tender and then to the open air through a large pipe, which can be seen in the photo. 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.

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