The first Hudsons of the Milwaukee Road were built by Baldwin between 1929 and 1931. While the first 14 locomotives were designated as class F6 and their running boards were broken above the air tank, the last eight were designated as F6a and had straight running boards. They were delivered with driving wheels with a diameter of 79 inches, which later increased to 80 inches due to thicker tires. The coffin feed water heater was apparently integrated into the smokebox for aesthetic reasons.

The boiler had a large firebox, which resulted in a load on the trailing bogie of around 53 tons. The express trains they hauled usually consisted of nine to ten cars, but often eleven to 14 cars and were occasionally even longer. With these trains, the 918-mile round trip between Minneapolis and Harlowton could be completed ten times in 30 days without any servicing stops. A lighter train covered a flat section of 65.6 miles at an average of 92.3 mph.