The Northern Pacific was the first US railroad to purchase a significant number of Mikados. These machines grouped under the class W were 55 locomotives of the subclass WA without a combustion chamber and 105 WB with a combustion chamber. Some of these were built as compound locomotives and later rebuilt to the simple variant. 40 class W-1 locomotives followed in 1910, which differed mainly in that they had Walschaerts valve gear in contrast to the Stephenson valve gear of their predecessors. Most of the locomotives were superheated from 1912, and the boiler pressure was increased to 200 psi with thicker boiler plates. 