2-8-8-4

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:

The equivalent UIC classification is, refined for simple articulated 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.

The Northern Pacific Railway was the first railroad to order a 2-8-8-4. The first was built in 1928 by American Locomotive Company; at the time, it was the largest locomotive ever built. It had the largest firebox ever applied to a steam locomotive, some 182 square feet (16.9 m²) in area, to burn Rosebud coal, a cheap low-quality coal. But the firebed was too large for the available draft and the fire burned poorly and developed under 5,000 horsepower. The problem was mitigated by blocking off the first few feet of the grates. Baldwin Locomotive Works built 11 more for the Northern Pacific in 1930. No examples have been preserved.

The Southern Pacific Railroad's famous "cab forward" articulated steam locomotives were effectively a Yellowstone in reverse (4-8-8-2). This was done to spare the crew from the heavy smoke output of the large engines on the former Central Pacific, where tunnels and snow sheds were common and lengthy. One is on display at the California State Railroad Museum in Sacramento. The ready availability of fuel oil in California made them possible. The SP also owned some conventional 2-8-8-4s for use in areas where coal was plentiful and snow sheds were rare. Lima Locomotive Works built 12 AC-9 class locomotives in 1939; they had skyline casings with striped pilots. At first, they burned coal but were later converted to oil. None were saved.

The Duluth, Missabe and Iron Range Railway hauled iron ore in Minnesota. Iron ore is heavy and the DM&IR operated long trains of ore cars, requiring maximum power. These locomotives were based upon ten 2-8-8-2s that Baldwin had built in the 1930s for the Western Pacific Railroad. The need for a larger, coal-burning firebox and a longer, all-weather cab led to the use of a four-wheel trailing truck, giving them the "Yellowstone" wheel arrangement. They were the most powerful Yellowstones built, producing 140,000 lbf (620 kN) of tractive effort, and had the most weight on drivers so that they were less prone to slipping.