Space between levels- around the wall

I'm sure some folks will come through with specific recommendations, but another method is to do some test setups yourself to try out different heights and widths of tables and shelves. Shim up a table to the height you are considering for the main layout, then rig shelves above that with boxes and blocking or whatever is handy. Set up some sections of track with cars on them on the different levels. Then you can see for yourself what looks best for your own height, standing and sitting. As a general rule, upper levels should be narrower than lower sections for better viewing and access.

Bob, on my around-the-wall layout I placed my upper levels starting against the wall or a few inches from it.  They are basically double track elevated lines that I will add scenery to.  For the most part, my middle level runs parallel to the upper level, but about 4"-6" in front of the upper level. 

The main level, where most of the scenery will go is 36" above the floor and 2' to 8' deep.  The middle level is 10" above the main level, the upper level is 20" above the main.  I chose those heights to be able to add mountain portals and still have some room for scenery (and my hand in case I need to reach in) above the portals.  Each of the upper levels are 8" wide.

Here are some examples of finalized parts of my continuously evolving track plan.  Bottom (main) level depth here is 4':

Here the upper levels cross over and under by using wider curves on the middle level. Space between levels will have hills with trees and foliage, rocks and/or mountain cuts or whatever I think of at the time I do it  :

Here's what 30" looks like...



The lower level is at 42" the upper is 72", but the floor is raised 12" 



The helix connects the levels





The upper level is usually not as deep as the lower, but in some places they are the same depth. 

It seems to work. I'm still thinking about lighting.

Eliot, wow! That is really well done. That's the best O scale helix I've ever seen. I'm not a big fan of double decker layouts but I think you nailed it with a home run.

I think that it isn't so much the space between the levels, but rather how high you need each level to be.  For instance, you don't want the have the lowest level be to low, otherwise is is difficult to maintain, and doesn't look as realistic.  The upper deck can't be too high, otherwise it will also be too hard to maintain (and see).  So I recommend having the upper level below eye level, and the lower level at or above waist level.

If that is a little to narrow between levels, then make hte upper level shallower, so the lower level extends outward beyond the upper deck.  This also aids in lighting because the lower deck is not hidden in the shadows and doesn't require as much light.

just an idea.

I agree with what James wrote above. I once saw a really great double decker 2 rail layout but the upper level was above (at least) my eye level. I had to strain on my tippy toes to see the upper level good. I'm 5'10" by the way. It would be even worse for someone shorter but of course much better for someone taller. In hindsight I would have placed the upper level a little lower. However, if the owner was happy with it where it was then that's fine too.