A 2-8-8-4 steam locomotive, under the Whyte notation, has two leading wheels, two sets of eight driving wheels, and a four-wheel trailing truck. The type was generally named the Yellowstone, a name given it by the first owner, the Northern Pacific Railway, whose lines ran near Yellowstone National Park. Seventy-two Yellowstone-type locomotives were built for four U.S. railroads.

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′.

A locomotive of this length must be an articulated locomotive. All Yellowstones 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 drivers of 68 inches (1.73 m) 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 depending on the criteria used.