In 1874, Swiss technical writer Anatole Mallet invented the articulated locomotive, coupling two sets of driving wheels under one boiler. Mallets used their steam twice: the rear cylinders were fed high-pressure boiler steam, and the larger front cylinders used low-pressure steam exhausted from the rear cylinders. In 1909, Baldwin delivered the first 2-8-8-2 Mallets to the Southern Pacific, where they became the precursors of the SP's famous Cab Forwards. By World War I, Mallet 2-8-8-2s were in use across the United States, particularly on mountain routes where they replaced two or three smaller engines.
When the USRA took over the railroads during the War, it used the Norfolk and Western class Y-2 2-8-8-2 as the starting point for the largest of its 12 standardized steam locomotive designs. (For more on the USRA, see page xx.) USRA designers turned the Y-2, which had been a disappointing design for the N&W, into a highly successful freight engine. Although Mallets later fell out of favor with most railroads, in favor of faster "simple" articulateds that delivered high-pressure boiler steam to all four cylinders, the USRA's largest design proved to be one of its most enduring. In 1952, a Norfolk and Western Y6b, a direct descendant of the original USRA design, was the last mainline steam engine built in the United States.
The USRA 2-8-8-2 returns to the RailKing lineup in 2014, upgraded with Proto-Sound 3.0 and wireless drawbar and featuring a full complement of Imperial-level details: legible builder's plates, painted backhead gauges, cab interior light, glowing firebox, engine crew, tender truck safety chains, and a real coal load in the tender.