When I first planned the double-deck long “Yantic-Willimantic” peninsula on my HO layout I concluded I needed to use a helix to move trains between the two levels.  Somewhere along the line I figured if I had the mainline gradually climb as it ran the length of the peninsula (about 36 feet or so) I could gain enough elevation to eliminate one or more turns from the helix.  But things started unravelling once I realized I had to (1) Leave some level track in the various switching areas – having sidings and spur tracks with grades can become an issue, so in general I wanted to keep these areas fairly level and (2) Once the railroad started climbing I was in danger of busting through the ground on the upper level before I wanted – in other words the separation between decks became too small.  

One way to get around this, of course, is to spiral the layout as it climbs – essentially maintain the separation between decks as both decks increase in height. This works if you have a true mushroom layout, with a raised floor, or if the starting elevation is fairly low. In my case, the base track level is about 47” at “water level” – New London harbor. Add 15”-16” to that and you end up with the upper deck about 63”-64” – which is about chin-height or a little less for me.  Having the upper deck fairly level at 63”, and having the lower deck climb to the entrance of the helix meant there was less and less visible “sky” on the lower level.  It was also obvious there would be some issues accessing the lower deck tracks for operating and even viewing. The answer was to level off the lower deck tracks and add another turn or two to the helix.

Another example of how multi-deck layouts are really a long series of compromises.