The Milwaukee Road's A2 class comprised 47 compound steam locomotives of the 4-4-2 or 'Atlantic' configuration. The Milwaukee Road acquired them in five batches.
The first two batches of 9 and 19 locomotives (classes A2 and A2-a) were built by Baldwin Locomotive Works, and were Vauclain compound locomotives with 84 inches (2.13 m) drivers. The third batch (A2-b) of five locomotive was built by the Milwaukee Road in its Milwaukee, Wisconsin shops with 85 inches (2.159 m) drivers. The fourth batch (A2-c) of 12 engines was built by Baldwin as Vauclain compounds with 85 inches (2.159 m) drivers, while the last batch (A2) was for a pair of engines built by Baldwin as balanced compounds. All members of the class were scrapped between 1927 and 1930.