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.
A locomotive of that length must be an articulated locomotive; all had a joint between the first and second groups of driving wheels. 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 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 these cab-forwards were built with simple expansion cylinders. The name stuck because the original classes of Southern Pacific cab-forwards were built as compound 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; the 145 engines built in 1930 and later had cylinders 24 inches by 32 inches, drivers 63-1/2 inches and boiler pressure 250 psi, giving a calculated tractive effort of 123,400 lb.
Southern Pacific 4294 is the only cab forward that survives today, at the California State Railroad Museum.