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 a Triplex of the Mallet type.
Other equivalent classifications are:
- UIC classification: (1'D)D(D1')
- 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
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.)