Think 2-8-4's tended to be eastern locomotives in that on the flatlands, they could pull all the tonnage that passing sidings would allow. In the west, those railroads needed much higher tractive effort to deal with mountain grades, so they began in the mid-1920's to employ 2-8-8-0's, 2-8-8-2's, 4-8-8-2's (SP cab-forwards) and 2-8-8-4's. 2-8-4's just could not meet their tonnage hauling needs in heavy gradient territory.
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