2-8-8-4

Under the Whyte notation for the classification of steam locomotives, a 2-8-8-4 is a steam locomotive with two leading wheels, two sets of eight driving wheels, and a four-wheel trailing truck.

Other equivalent classifications are:

• UIC classification: 1DD2 (also known as German classification and Italian classification)
• French classification: 140+042
• Turkish classification: 45+46
• Swiss classification: 4/5+4/6
• Russian classification: 1-4-0+0-4-2

The equivalent UIC classification is, refined for Mallet locomotives, (1′D)D2′.

Such a long locomotive must be an articulated locomotive, and all the examples produced were of the Mallet type, having a hinged joint between the first and second groups of driving wheels, and having the superstructure of the locomotive rigidly attached to the rearmost set, with the forward set and leading truck allowed to swing laterally on curves.

The type was generally named the Yellowstone, a name given it by the first owner, the Northern Pacific Railway, whose lines run near Yellowstone National Park. Seventy-two Yellowstone type locomotives were built for four different US railroads.

The 2-8-8-4 was the common choice of arrangement for the very largest steam locomotives at moderate speeds. All classes of Yellowstone had fairly small drivers of 63 to 64 inches (1.60 to 1.63 m). For greater speeds, the Union Pacific Railroad chose a four-wheel leading truck and 68 inches (1.73 m) drivers for its Big Boy 4-8-8-4 class.

Several classes of Yellowstone, especially the Duluth, Missabe and Iron Range's locomotives, are among the largest steam locomotives, with the exact ranking dependent on the criteria used.