Andy: The cars look GREAT! What a super job. I noted the Marx "Pickle Fork" coupler on the rear of the observation, did you use those throughout? Do you plan on using a Marx Loco to pull the cars? If you do I bet a Marx 333 4-6-2 loco available from '49-58 would be great, its die cast and a great runner. The 666 2-4-2 from '55 also die cast is another possibility. Finally the 1829 a Hudson 4-6-4 while plastic has a lot of detail in the casting and is really sharp.
Well, anyway what a terrific job best of luck with them!!
Don