Not exactly sure of what you are saying?
Yes, was a bit clear as mud wasn't it. What I was meaning (and the main reason for that long train video from the club, and the layout you've shown as an example) it's possible to have more than one mainline without necessarily going to having them built on seperate shelves, accessed by a helix. They can still be at different levels but have interchanges with each at common yards. With a smaller layout of course, you will be restricted to one, or maybe two such interchanges. You can still, as at my club, use a helix to get a little more elevation between the mainlines if you wish. Many of those scenes on that video are less than 18" wide.
The spiral's mountain is hollow shell, but that still has to be supported by framing and that is what creates the difficlties with tunnels and tracks underneath. In the case with the club's one, the tunnels beneath were an afterthought, many years after it was built, so there was no forward planning involved. One track has a dip in it to clear a support, which can and does at times create uncoupling issues in a spot which is hard to get to. If it had been built originally from the bottom up, the top exposed track's supports would have been properly placed.