Thinking of a helix?

[ednadolski]

Theoretically one need not make this from plywood at all. Zero waste, just alternate the miter cuts on a 1x4 or 1x6.

You could also rip the plywood into 1x4 or 1x6 (or other size) strips.   The 3/4" would be better if you want to use a biscuit joiner.

Edit:  Biscuit joints would probably go faster and take a lot less clamping and gluing than with the overlapped sections.

Ed

[CRL]

The disadvantage I see in the one piece approach is the inability to correct small alignment errors as you go up. Not an issue if your mitre cuts are perfect, but good luck with that.

[LIRR]

When I first planned my railroad many years ago it included a helix. After considering several different methods I decided the best way forward was to omit it. Turned out to be the right call. I had plenty of railroad to build, scenic and maintain, and plenty of operations, without the second deck. If a helix work for you, may the Schwartz be with you.