I’m afraid the helix idea has hit the dust. It is just too large in my limited layout space (about a ninth of the room floor space!).
Instead I’m thinking of going for a vertical traverser that will be 4′ long within a reversing loop so I can ensure that locos run on and off facing the correct direction:
The balloon track will run under the loft eaves (29″ in) and I may take the scenery out in that corner as a kind of shadow box – I will have to see how it looks. The traverser section is marked in red and can be behind some scenery/backdrop to hide it.
The vertical traverser design is ongoing but will probably have door sliding gear for the vertical supports and we’re thinking of a mechanical winding mechanism with an old fashioned type wheel for character. That bits still up in the air but I have a friend with an engineering mind off to scrap yards this weekend to see what he can find that might suit.
It may not be a continuous run like a helix and so is slightly more difficult but it will be more accessible and easier to hide scenicly in my layout.