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. Because of its length, such a locomotive must be an articulated locomotive. It is not 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.
Other equivalent classifications are:
UIC classification: 1-D-D-D-1 (also known as German classification and Italian classification)
AAR classification: 1-D-D-D-1 French classification: 140+040+041
Turkish classification: 45+44+45
Swiss classification: 4/5+4/4+4/5
The equivalent UIC classification is to be refined to (1'D)D(D1') for these Triplexes.
Baldwin built the only three examples of the type for the Erie Railroad between 1914 and 1916. The first was named Matt H. Shay, after a beloved employee of that road. It could pull 650 freight cars. All three, as well as the lone 2-8-8-8-4 and several Virginian Railway electrics, shared the nickname "Triplex" because of their three sets of drivers. (Compare Duplexes, which had two sets.)
An overview of Triplex engineering is given at Triplex (locomotive)