You can use regular sectional track and solder them together.
You could make your own.
Since this is hidden track and asthetics is not a concern, you could make your own roadbed by cutting a piece of 1/4 inch plywood into the radius you need. 1.5 inches wide. Maybe use Peco track since it holds its curve and is easy to mold. Then mount the peco sections onto the plywood. You could use silicon adhesive caulk.
Maybe use the ribbon rail guide as the template with which to draw your lines on the ply, and to form the Peco track.
It would form several very solid sections, or one larger section if you're ambitous, of very solid track and roadbed that you could place anywhere.
- Douglas
Skip the roadbed in a helix. It is not necessary and it takes extra time to install it and the extra height cuts down on the overhead clearance.
Jim it looks like you are going to have to go with flex track for the curves.
I laid mine out on a table with the centerlines drawn and soldered them together in a curved shape after I cut the ties and rails to fit. I installed them on the helix levels as I went. Tedious but not difficult, especially if you buy the prefab helix kit. I used atlas code 100.
Guy
see stuff at: the Willoughby Line Site
I opted to skip the roadbed, no cork or foam, inside my helix 8 years ago. However, changes of temps and humidity in the attic above the garage in which my layout sat meant that I had to control stringlining rails, pop-outs, buckling, and other mishaps...especially inside that monster helix-that-was-a-mountain.
So, no roadbed, but I used DAP Alex Plus with Silicone caulking, the clear kind because it worked a LOT better than their 'white' version (at least, that's what I learned) to adhere the rails in place. This worked very well. I never had any problems inside that helix except for a single stringline incident with a long consist. None of the joints, including full gaps across sections of the sub-roadbed designed to help disassemble and move the apparatus, ever shifted.
What helped to keep the wood, and therefore the joints, stable was that I controlled humidity in the loft by means of a dehumidifier.
Bachmann code 100 EZ Track goes up to 35.5" radius.
https://shop.bachmanntrains.com/index.php?main_page=product_info&cPath=258_366_367&products_id=2462&zenid=99cpn01rp2rfo74lnbmnqjv3e2
FWIW the largest minimum radius I recall ever seeing for an HO product was a brass CP 2-10-4 whose builder recommended 30"R curves. However, an MRN reviewer said it had no problem going around 28.75" Kato Unitrack. So,as has been mentioned, using the 31"R Kato code 83 track could be a good substitute.
Stix,
I have a couple of brass cab forwards that won't do anything less than 30" - Since they were released many years ago, I don't know what the paperwork that came from the factory might have specified since it was long gone by the time I purchased them. My tests showed the locos to very sensitive to anything even slightly less that 30".
My last suggestion sucked.
How about this.
Take bendable track like Peco. Use ribbon rail to form a precisely formed curve, if necessary. Then just brush glue onto the ties and spikeheads, underneath too, and it should form a fixed three foot section of curved track.
Bachmann sells 35-1/2 inch radius EZ track: https://shop.bachmanntrains.com/index.php?main_page=product_info&cPath=258_366_367&products_id=2462&zenid=m17h13aj6gktgqbl7deekelkg2