Pitchwife, Whatever material you use for the helix roadbed, you will have to allow for splice joints as I don't know how you can avoid having to join pieces together. This, even with your 1/4"plywood, would require another 1/4" splice plate, so now you are at 1/2". I used 3/4" and routed as described above to join, you could save 1/4" using your method, just be sure that you won't have warpage problems. I have never liked using 1/4" plywood for roadbed. I definately wouldn't advise routing half the thickness of 1/4" material as I did with the 3/4". Regarding cheating the NMRA clearances, you certainly want to be aware of exactly what you are limiting yourself to. In my case, the helix is used for interchange between two railroads located on different levels. I wanted, as a minimum, to be able to run standard 50's era freight cars on the helix. No hi cubes on my railroad! so I was able to cheat down to 2 5/8", including the track height. No, I will never be able to run modern stuff if I change my mind, but I can assure you that will never happen. I did buy Accurails auto racks, I can run them on the helix but only without autos on the top deck. As far as Woodie's statement about room to fix derailments, I do not even attempt to rerail equipment that derails(thankfully, it doesn't happen often!). I simply remove the offending piece of equipment from its train, couple the train back together, and resume its run. The problem car is inspected and if ok, returned to its train after exiting the helix. Given that this process, while not really difficult, does interfere with the enjoyment of operating, I recommend you build a helix from good sturdy materials and put the effort into designing it to be dimensionally stable. I repeat my advise about tracklaying on the helix. Using flex track, stagger all rail joints, solder every joiner while last foot or so of flex track is still straight, make sure rail is in cast on spikes, not resting on top of them, so on. You will not get a second chance without dismantling the helix. Going thru the effort of building the helix and having trains constantly derail is going to bum you out. Also remember to be generous with power feeds, although if you have soldered each joint, this shouldn't be a problem. On trackage as hard to work on as a helix, there is no such thing as overkill. Be meticulous in your workmanship once, then don't worry about it anymore!
Gary