One of my good friends is doing a new layout with a truly mammoth helix - but his son is a CAD whiz as well as a modeler, and laid the entire thing out and laser-cut it for assembly. It's a four-track monster (two up and two down) that connects two levels, and basically, for all intents and purposes, works as a staging yard in itself. The lower level and the upper level are completely functional and open layout areas with about 12-14" between them (guessing). A visible classification yard is on the lower level, massive enough, and the 'upper' level is mostly wide-open running track around the room with a lift-out. Upper level is at a good 60". Each level is also designed to work independently so that running the helix is optional - you can run out of the helix and return to it on each level.
What impressed me about that idea is that given the radius and grade of the helix - and the fact that it's big enough to climb up into in the middle like a gun turret on a tank (access from the bottom, with handgrips and everything!) - is that multiple parallel tracks on a helix didn't cost much space, and with the 'down' tracks on the INSIDE of the helix (tighter curve) than the up tracks (wider curves) the payoff was a good trade between using all that wasted space for train storage without having every train have to climb up that thing because it functions as a staging yard. And it's big enough to hold trains end-to-end in it's own blocks, so he can pack in 6-8 trains in there. Open curved panels on the outside let you monitor it and then close it up.
I have to admit that impressed me, because one of the 'dumb things' about helixes is taking a coffee break while a train from a lower level storage yard climbs out of the hole, by the time you get there it's 4X the time of the visible layout. So making the helix itself the multi-track staging yard between visible levels has been done, I've seen it. I've seen lots of single-track and 2-track ones, but a 4-track monster was interesting.
Jerry Britton's PRR N layout (dismantled out of frustration with bad switches) had an epic helix and an enormous staging yard almost at floor level maybe as low as a foot. I think the helix was two track, and it took a LONG time to climb up to one of several visible levels, time enough you could go upstairs and grab a snack and make it back down by the time you could actually see a train.
