The Milwaukee Road's class "F7" comprised six (#100–#105) high-speed, streamlined 4-6-4 "Baltic" (Hudson) type steam locomotives built by the American Locomotive Company (ALCO) in 1937–38 to haul the Milwaukee's Hiawatha express passenger trains. Following on from the success of the road's class "A" 4-4-2s, the F7s allowed the road to haul heavier trains on the popular Chicago–Twin Cities routes. In the late 1940s, the Milwaukee F7s were modified to equip an additional Mars Light above the original single highlight to further enhance the safety of daily highspeed operation.