Is the difference in sound really that much between solid rail track and traditional tubular? I wouldn't think it would make much difference when you're screwed down to a big piece of wood in open air that will resonate the sound anyway.
If it were me, for track that you can't see anyway, I would use Traditional tubular, and probably used track since it doesn't matter if it's not shinny, though new Menards track is less expensive than used track for wide curves. Stop by a carpet store and see if they have any scraps a few inches wide that you can use as roadbed to pretty much kill any sound. If not, just buy some at a home improvement store.
Just don't see the point in expensive track that will never be seen.
JGL
