N Scale Skytops specialized in resin shells for Milwaukee Road Hiawatha/Olympian "skytop" observation cars. Said shells were designed to be mounted on American Ltd passenger car "core" kits. Three different carbody styles were available -
Creek Series Sleeper -
Rapids Series Parlor -
As of this 2013 writing, N Scale Skytops is no longer in business. However, Bill Denton (Skytop Models) was at one time selling improved versions of these castings, and (depending on when you're reading this) may be able to hook you up.