Chanda said:
The plywood that is left is where you put your scenery. I admit that plywood is out of sight but the same method can be used if you use MDF which is cheaper.
I'd avoid MDF if possible, as even in 3/4" thickness it won't support its own weight over any distance without sagging. It is also difficult to drive track spikes into it, and it is very poor for holding either nails or screws. Depending on how you frame the support for the plywood, 1/2" or 5/8" should be adequate, and even if you use 3/4" plywood, sheathing grade, which is usually spruce, will work fine. There's no need to buy fir ply, which is usually sold as "good one side", with the other side so-so, or good both sides, and with a price to match. For our needs, sheathing plywood, which has both sides unsanded, is good enough strength-wise, and if you're not nuts about its appearance, it's an added incentive to get your scenery started.
Wayne