2-8-8-2

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A 2-8-8-2, in the Whyte notation for describing steam locomotive wheel arrangements, is an articulated locomotive with a two-wheel leading truck, two sets of eight driving wheels, and a two-wheel trailing truck. The equivalent UIC classification is (1'D)D1'. These locomotives usually employ the Mallet principle of articulation, with a swinging front engine and a rigidly attached rear engine. The 2-8-8-2 was a design largely limited to American locomotive builders.

The first 2-8-8-2 was built in 1909 by Baldwin, who sold two to the Southern Pacific Railroad (classified MC-1), and then three each to the Union Pacific and UP-owned Oregon River and Navigation Company. Baldwin conceived the type as an expansion of the 2-6-6-2 permitting a greater tractive effort.

The next order for the type was from the Southern Pacific; these differed in being cab forward locomotives, so that the crew could have better visibility and breathing in the SP's long tunnels and snowsheds. They were very successful, and SP continued to order cab-forward locomotives, building an eventual fleet of 256 of numerous classes; later cab-forwards were 4-8-8-2s.

The 2-8-8-2 proved itself to be a capable hauler on mountain grades, enabling the replacement of several smaller locomotives and hauling longer trains than before. They were not fast; they hauled at drag freight speeds, up to 25 mph. The locomotives were adopted by a broad spectrum of mountain railroads, including the Norfolk & Western, Southern, Virginian, Clinchfield, Denver & Rio Grande, Reading, Western Maryland, Missouri Pacific, Frisco, and the Duluth, Missabe and Iron Range Railway. On many railroads, the locomotives of the type were the most powerful on the roster. When built, the 2-8-8-2s of the Western Pacific Railroad were among the most powerful steam locomotives in the world and formed the basis for the later 2-8-8-4 "Yellowstone" type engines used by the Duluth, Missabe and Iron Range.