To replace the Pacifics on the Twin Cities Hiawatha, the Milwaukee Road introduced a new type of Hudson or Baltic locomotives in 1938. The six locomotives were built by ALCO-Schenectady, received the numbers 100 to 105 and were designated class F-7. Their streamlined casing was designed by Otto Kuhler.

They were similar to the Chicago & North Western's class E-4, but had differences in the boiler and the cylinder dimensions. Their boiler was similar in size with some 4-8-4s and might have generated a power of around 5,000 hp. With their 84-inch drivers, they regularly travelled longer distances with more than 100 mph. Speeds they actually reached amounted to 120 mph, with up to 125 mph over short distances being claimed.

On the Hiawatha, they reduced the travel time for the 412 miles between Chicago and the Twin Cities of Minneapolis and St. Paul to six and a half hours. There they had to do average speeds of more than 80 mph for an hour to keep up with the schedule. When the Twin Cities Hiawatha was dieselized, they were used on other Hiawatha trains. After a short career, they were scrapped between 1949 and 1951.