The Milwaukee Road's class "F7" comprised six high-speed, streamlined4-6-4 "Baltic" type steam locomotives built by the American Locomotive Company 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 daily. One run inJanuary 1941 recorded by a reporter for Trainsmagazine saw 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 and sustained an average for. However, the British locomotive LNER Class A4 4468 Mallard is officially accepted to be the world's fastest, with a run recorded at but authenticated at 126 mph in 1938. The Mallard run was slightly downhill and the locomotive broke down at the end of the run. 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 Hiawatha schedule was modified such that the engines would need to run the between Portage and Sparta, Wisconsin in 58 minutes—a start-to-stop average of. 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. 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 froze in its guide. It immediately overheated, broke, and dropped from the guide while the train was traveling at an estimated speed between 90 and over 100 mph. Air brake lines were severed, putting the engine into emergency. The engine was severely damaged, broken drive gear tore up ties and roadbed, and debris was found as far as 1400 feet west of Edgebrook Station. Information is conflicting on the amount of injuries that resulted; some report that two railroad employees were injured. Another report stated that as the locomotive passed by the Devon Avenue crossing, an automobile driver was injured by flying debris. Whatever the case, no one was killed. The train stayed on the rails, and continued to over 10,560 feet from the station until coming to a complete stop. The cause of the incident was later found to have been caused by the failure of a connection link between the valve gear's combination lever and a Nathan mechanical lubricator. In fact, both of the locomotive's crossheads had been running dry on lubrication, but the right one was the first to fail. After this incident, #102 never ran again, as the cost was not considered worth repairs. 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 of them were scrapped.
