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.

Milwaukee Road class F7
A class F7 being moved out of the Alco factory
Type and origin Power type Steam Builder American Locomotive Company Serial number 69064–69069 Build date August to September 1938 Total produced 6
Specifications Configuration: ​  • Whyte 4-6-4 Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge Leading dia. 36 in (914 mm) Driver dia. 84 in (2,134 mm) Trailing dia. 38 and 44 in (965 and 1,118 mm) Wheelbase Loco & tender: 89 ft 10 in (27.38 m) Length 100 ft 0 in (30.48 m) Axle load 72,250 lb (32,770 kilograms; 32.77 metric tons) Adhesive weight 216,000 lb (98,000 kilograms; 98 metric tons) Loco weight 415,000 lb (188,000 kilograms; 188 metric tons) Total weight 791,000 lb (359,000 kilograms; 359 metric tons) full Fuel type Coal Fuel capacity 50,000 lb (23,000 kilograms; 23 metric tons) Water cap. 20,000 US gal (75,700 L; 16,700 imp gal) Firebox: ​  • Grate area 96.5 sq ft (8.97 m2) Boiler pressure 300 lbf/in2 (2.07 MPa) Heating surface 4,166 sq ft (387.0 m2) Superheater: ​  • Type Type E  • Heating area 1,695 sq ft (157.5 m2) Cylinders Two, outside Cylinder size 23+1⁄2 in × 30 in (597 mm × 762 mm)
Performance figures Maximum speed 120 mph (193 km/h) Tractive effort 50,294 lbf (223.72 kN) Factor of adh. 4.29
Career Operators Milwaukee Road Numbers 100–105 Retired November 1949 to August 1951 Disposition All scrapped

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 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 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]

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 (including the main rod) was found as far as 1400 feet west of Edgebrook Station. Information is conflicting on the number of injuries that resulted: Some report that two railroad employees were injured, while 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.^[2] 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. None survive today, as the last one was scrapped in 1951.