Just my opinion, but I think the only realistic option is to pull finished interiors out of other passenger cars, and transplant them, doing whatever cutting, etc., that's necessary. I've taken Lionel 15" aluminum passenger cars with finished interiors, removed the shells, and put the shells from the set of cars I want onto the chassis' that had the finished interiors. The chassis and interiors are interchangable on Lionel's 15' aluminum cars, going back to the MPC days, on up to recent times. No cutting is necessary.
Not cheap, because you have to buy another set of cars with finished interiors, but that's the only realistic way to do it, I think. I put the unused shells (with the silhouettes) and chassis' together and then sell them, so that helps recoup some of the cost. The hardest part of the procedure is removing the tinted clear windows from the shells of the finished interior set, without damaging them, as they are glued in. The silhouette strips just slide in and out. If I do this procedure again, I may try to find some tinted plastic sheets that I can just cut and slide in.