In the photo above, are the two levels “stepped” so you can sit in a chair and operate the inside on the bottom level but there is a raised floor so you can sit and operate the upper level on the outside? If so, this is a great design feature and one we’ve been seeing a lot in recent years. Remember onewolf is doing that same type of design, although not meant for rolling chair operation, you do stand at different levels so each scene is mostly self contained. I think the term I’ve seen used by some is “mushroom”.
A goodly part of my test track/layout goes around two walls of a room (12 x 14). It’s a foot wide (except in the corner–48" radius). Support is by 1/2" lag bolts run into the studs. There is nothing projecting below. There is an aluminum fascia strip 2" high that defines the edge. And it provides room for the various block switches.
And if/when the structure is removed, there’s only 1/2" holes to patch.
Where it crosses doors and windows, there are liftouts. Should the need arise. Which it hasn’t.
No step (other than on the stepstool seen in the photo), as the upper level is meant to be operated while standing in the same aisle that the chairs sometimes occupy. Because I operate the layout alone, I simply push the chairs or stepstools out of the area where I’m running a train.
The stepstools are there mainly for use when I’m working on the upper level…laying track or adding scenery or structures. For train operation there, everything is accessible while standing…no need for the stepstools (except, perhaps, for the younger grandkids).