The Milwaukee Road classes F6 and F6-a comprised twenty-two steam locomotives of the 4-6-4 configuration, commonly nicknamed “Hudson” but known as “Baltic” on the Milwaukee Road. The fourteen class F6 locomotives were not delivered from their builder, the Baldwin Locomotive Works of Philadelphia, Pennsylvania, until 1929–1930. In 1931, eight sister locomotives of class F6-a were delivered; these differed in few aspects but can be distinguished by the straight running boards of the F6-a, in contrast to the stepped running boards of the F6.