On my Con-Cor passenger cars, I used a combination of things. I got a IHC interior kit for a Rivarossi dome car, but found it didn't fit. I cut out the upper dome seating area and installed that in the dome and filled the dome with people. I painted shades drawn in the area under the dome where I didn't have room to model an interior. At both ends of the car, I used styrene sheet to model bulkheads and I used Pikestuff coach seats. In fact I bought a few packages of the Pikestuff seats and used styrene to add arms. When I wanted to make a single seat, I cut the Pikestuff seat in half, and put styrene arms on either side. In the dining car, a piece of styrene cut to shape and glued to the laps of diners sitting in my converted Pikestuff chairs, makes a believable dining table. Most passenger car windows are small enough, that a view block to keep you from looking all the way across the car, and a few chairs to simulate roomettes with a few passngers either sitting, or with legs cut off to simulate seating will look good.