The Milwaukee Road's A2 class comprised 47 compound

Compound locomotive

A compound engine unit is a type of steam engine where steam is expanded in two or more stages.A typical arrangement for a compound engine is that the steam is first expanded in a high-pressure cylinder, then having given up heat and losing pressure, it exhausts directly into one or more larger...

 steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

s of the 4-4-2

4-4-2 (locomotive)

Under the Whyte notation for the classification of steam locomotives, 4-4-2 represents the wheel arrangement of four leading wheels on two axles , four powered and coupled driving wheels on two axles, and two trailing wheels on one axle...

 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

Baldwin Locomotive Works

The Baldwin Locomotive Works was an American builder of railroad locomotives. It was located in Philadelphia, Pennsylvania, originally, and later in nearby Eddystone, Pennsylvania. Although the company was very successful as a producer of steam locomotives, its transition to the production of...

, and were Vauclain compound

Vauclain compound

The Vauclain compound was a type of compound steam locomotive that was briefly popular around 1900. Developed at the Baldwin Locomotive Works, it featured two pistons moving in parallel, driving a common crosshead and controlled by a common valve gear using a single, complex piston...

 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

Milwaukee, Wisconsin

Milwaukee is the largest city in the U.S. state of Wisconsin, the 28th most populous city in the United States and 39th most populous region in the United States. It is the county seat of Milwaukee County and is located on the southwestern shore of Lake Michigan. According to 2010 census data, the...

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.