The Class A2 designated the first Atlantics on the Milwaukee Road to have a Vauclain-type compound engine. This means that there was a high and a low pressure cylinder on each side, which were connected to a common crosshead. The trailing axle had special inside bearings developed by J.H. DeVoy that were part of a large single casting.

Nine A2 and 15 A2a were built by Baldwin in 1901 and 1902 respectively. The five A2b had drivers whose diameter had been increased to 85 inches. While these were built in the railroad's own workshops in Milwaukee, the twelve A2cs were built again at Baldwin. These were all scrapped between 1927 and 1930.

An exception were two locomotives that were built in 1907. These had what was known in the USA as a “balanced compound” drive, which had separate inside and outside cylinders. These locomotives were rebuilt in 1938 to class A-4a with simple expansion and remained in service until 1951.