After the K-2 class Pacifics, which did not have a superheater from the factory, the New York Central had a larger number of superheated K-3 built between 1911 and 1925. Their task was to pull the heaviest express trains. Numerous series from the K-3a to the K-3r were produced, with the latter being built for the Big Four. The different series were almost identical in the basic dimensions, but there were differences in the boiler. Some of the locomotives had a booster on the trailing axle. 