The Milwaukee Road's class F7 comprised six (#100–#105) high-speed, streamlined 4-6-4 "Baltic" or "Hudson" type steam locomotives built by 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.

The F7s are major contenders for the fastest steam locomotives ever built, as they ran at over 100 miles per hour (160 km/h) daily. One run in January 1941 recorded by a reporter for Trains magazine saw 110 miles per hour (180 km/h) achieved twice—in the midst of a heavy snowstorm. Baron Gérard Vuillet, a French railroading expert, once recorded a run between Chicago and Milwaukee where the locomotive reached 125 mph (201 km/h) and sustained an average 120 mph (190 km/h) for 4.5 miles (7.2 km).^[1] However, the British locomotive LNER Class A4 4468 Mallard is officially accepted to be the world's fastest, with a run recorded at 125.88 mph (202.58 km/h) but authenticated at 126 mph in 1938.

The Milwaukee F7s are accepted as the fastest steam locomotives by a different measure—scheduled speed between stations. In 1939, shortly after they were introduced into passenger service, the Twin Cities Hiawathaschedule was modified such that the engines would need to run the 78.3 mi (126.0 km) between Portage and Sparta, Wisconsin in 58 minutes—a start-to-stop average of 81 mph (130 km/h).^[1]

On July 27, 1950, F7 #102 was on a run between Chicago and Milwaukee on the "North Woods Hiawatha." 73 miles from Milwaukee, the right main crosshead overheated, broke, and dropped from the guide while the train was traveling at an estimated speed of over 100 mph. The engine was severely damaged, broken drive gear tore up ties and roadbed, two railroad employees were injured, and debris (including the main rod) was found as far as 1400 feet west of Edgebrook Station. The train itself continued to over 10,560 feet from the station until coming to a complete stop.

The first one built, #100, was also the first withdrawn from service, on November 10, 1949; and the last one built #105 was the final one in service and was withdrawn August 10, 1951. All were scrapped.