can anyone tell me what thickness of styrene to use for flooring in athearn heavyweight passenger cars?
You can figure it out. The floor should be at the same level as the floor in the vestibule. On my Athearn BB heavyweight Pullman, that means you should use very thin styrene - .010", maybe - since the floor is all at the same level. You’ll have to move the weights, of course, to the roof. Not good, but you can also add some in the battery box openings in the floor. I would say that you should test a couple of the seats on various thickness of styrene and see what looks good through the windows. You’ll never see the floor from the outside of the car, anyway…
If you’re going to add interiors, you could look into Palace Car Co. interiors. Their interior kits come with a floor piece made of metal so it adds weight, so you probably wouldn’t need to use the Athearn weights, and is designed to drop into the car.