During 1947-48, the Milwaukee Road ordered 72 all new coaches for the Olympian, Twin Cities Hi, Midwest Hi, Chippewa and general passenger pool. Although all built in the west Milwaukee shops of the railroad, the cars were not identical. While the window arrangements were the same, the cars had two different air conditioning systems. The differences can be noticed on the roofs and also on the underbodies  The Waukesha air conditioning system can be recognized by the crowded underbody containing the air conditioning unit and propane bottle carrier. In contrast, the steam ejector system had a rather plain underbody but extra hatches and vents on the roof.  The cars were origiinally built with either 48 or 52 seats and numbered #515-551 but some were later re-equipped with 40 leg rest seats and numbered into the 600 series.