If you have the room, I'd suggest going with a modelled grade, rather than a helix - why would you want to waste the scenic opportunities, not to mention hide expensive track and not be able to watch your trains climb the grade?
My layout is set up to be partially double-decked and the climb up the grade will be an interesting operational challenge. The top surface of the second level will be 23" above the top of the layout below, and the same width, varying from 21" to 36". The extreme height differential is to accomodate fluorescent light fixtures fastened to the underside of the upper deck - otherwise the lower level will be too dark.
I run short trains, but most will be doubleheaded, and some may require a pusher, too. I wouldn't want to hide that kind of operation from view.
In the drawing below, the green area is single level, with the grade to the second level starting just below the word "Cayuga", at right, and running around the perimeter of the peninsula until it reaches the top where the grey begins (above the word "Elfrida").
In this photo, South Cayuga is out-of-sight to the left, and the end of the peninsula off-camera to the right. The top of the grade is behind those pointy-looking mountains in the centre distance.
This view shows the distance between levels, with the current end-of-track (the piece of roadbed hanging out in mid-air over the tunnel portal.
The bulging backdrop is to conceal a drainpipe and an outside corner of the room. The opening will allow me to fasten the partially assembled framework to the wall. The upper level (and the underslung light fixtures) will be supported by welded angle iron brackets, bolted to the wall studs.
Wayne