4-8-8-2
Under the Whyte notation for the classification of steam locomotives, a 4-8-8-2 is a locomotive with four leading wheels, two sets of eight driving wheels, and a two-wheel trailing truck.
Other equivalent classifications are:
UIC classification: 2DD1 (also known as German classification and Italian classification)
French classification: 240+041
Turkish classification: 46+45
Swiss classification: 4/6+4/5
The equivalent UIC classification is refined to (2'D)D1' for Mallet locomotives.
Because of its length, such a locomotive must be an articulated locomotive. All of the examples produced had a hinged joint between the first and second groups of driving wheels. Furthermore, all examples of this type were cab forwards. Normally, the leading truck sits under the smokebox and the trailing truck under the firebox. On a cab-forward, the leading truck also supports the firebox, and the trailing truck and smokebox are at the rear next to the tender. A 4-8-8-2 is effectively a 2-8-8-4 that always runs in reverse. Although commonly called Mallets they were not of this type, because unlike Mallets, which use compound expansion, these cab-forwards were built with simple expansion cylinders. The name stuck because the original classes of Southern Pacific cab-forwards were built as Mallets, though these were also eventually converted to simple expansion.
The Southern Pacific was the only railroad to operate engines of this wheel arrangement. All were built by the Baldwin Locomotive Works.
One example, Southern Pacific 4294, survives. It is kept at the California State Railroad Museum.