Here's what Bill Denton (Skytop Models) has to say about these resin kit cars -
Passenger car castings include the basic body shell along with a separate underframe casting. Modelers will generally need to purchase trucks, couplers, diaphragms and miscellaneous items separately. I have tried to make each casting as accurate as possible with most details already cast in place allowing the modeler to build highly detailed and very railroad specific models with a minimal amount of effort.
Three different carbody styles are available -
Milwaukee Road "Valley Series" Parlor -
Milwaukee Road "River Series" Sleeper -
CB&Q Havelock "Economy" Baggage -
Bill also sells improved versions of the old Jim Stillman (N Scale Skytops) Milwaukee Road skytops -
Milwaukee Road "Creek Series" Lounge -
As of this 2019 writing, Bill has started to dial back the amount of time he is devoting to his model business. So, if your idea is to acquire something from him, better to do it sooner rather than later.