Two thoughts

My first thought is "Wow, what a great and unique idea!"

My second thought is that spline roadbed might take up too much vertical space.  Depending on how you do it, I think a plywood helix could be made a little thinner, reducing the grade

I wouldn't go there

I would be hesitant to use spline in a helix.  First, as Ken mentioned, it is frequently thicker than plywood which wastes space and/or forces steeper grades in the helix.  The second thing is, unless you make it very wide, you will have no safety area on the inside of the curve in case of derailments or stringlining; stuff will just head straight for the floor if it comes off the track.  It's also not trivial to keep the radius of spline consistent, which is pretty important in getting a helix to perform reliably.  On top of that you will only be able to build a bit at a time because of the need to smooth the surface before being able to lay track, which you would not be able to do with another layer over top.  You will also need to build extra outriggers to attach the supports to it.  Lots of complications and no real advantages.

I've only seen one helix built out of spline and it was a mess.  I really don't think it's a good way to go. 

Helixes. Helices. More than one helix.

There's a lot of good theory and practice on building a helix at http://model-railroad-hobbyist.com/magazine/mrh-2012-01-jan/up_the_creek

He carries on the story through columns in the following months.

Getting consistent curves

As Jurgen pointed out, getting a constant radius might be difficult.  This is how I would try it:

I would build a form for 1/4 or 1/2 of the helix, and lay up at least 2 strips, preferably 3.  Ideally, I would make two forms so that I could make mating pairs.  The form(s) would ideally have the grade built in.  After building enough pieces, I would start laying them up using the pre-fabricated parts to hold a consistent radius.

For support, I would consider using vertical strips, or ideally a solid sheet, on the inside of the helix.  Either one could be glued into place after the roadbed was glued up and finished, allowing you to carefully tweak the grade as you go.  Speaking of sheets, a fabric sheet attached to the outside with Velcro would prevent a derailed car from taking a trip to the floor, but still allow access if needed.

If you can really get good results out of 1/2" tall splines, then that would solve the thickness issue when compared to plywood.  My take on the whole idea is that it could be done and could work, but it would take a lot more preparation and planning to pull it off.  Once the prep was done, however, it should come together nicely and might be easier than a traditional helix in some ways.

Spline Helix

Thanks for your ideas especially about the form having the grade built-in as I was worried about the flexing of the spline if it was originally built flat.  I have a semicircle of 18"R that I was playing with today that was built flat and it flexed a little when I was seeing what would happen if I made one end an inch higher than the other (as you know a little is a lot in N-scale).  I have seen the same happen to 3/8" plywood that I have used for curves over the years and had to put in extra risers to hold it in the right place.

Since this is N-scale, one of the concepts that I am considering was using 1/2" tall splines semicircles that would be built on plywood forms to control the radius as close as possible.  The semicircle would be joined with a 9 to 12" straight section (maybe made from pine planed to 1/2" or 1/2" plywood) and then the other semicircle would be added to form the oblong helix shape.  I'm thinking that I might look into the possibility of joining the spline semicircle to the straight section with #10 or #0 biscuits.  The article about Charlie Comstock's new layout that has been in MRH magazine over the last few months gave me the idea about using the biscuits. I do a lot of cabinet building with biscuits so I'll see what happens when I try to use them this way.

John Tanzillo