By my own calculations, the helix (spiral) would have to be at least 6.5 feet in diameter, to permit a minimum of 7" between successive turns. You would need 5 1/2 turns to achieve the desired 36" difference in height. The grade would average about 2.8 percent. That's as steep as I would go personally, and I don't advise using any curves sharper than O72. This is for a very basic single-track helix. There are more complex variations with two tracks, sidings to store a train out of sight, etc.
I believe @Susan Deats has a spiral like this, and I believe hers is also suspended from the ceiling. Maybe she will chime in and share some details of its construction. Check out this link provided by Dale Manquen:
A popular way to support successive layers of the helix, is to use threaded rod. This allows adjusting the height of the sub-roadbed in precise increments. Good luck with your project!
