I've also been installing guardrails on the hidden mainline portions. For some of this, I'm using the plastic cloth-like stuff used for weaving yard furniture chairs and such; it's bright yellow and pretty easy & cheap to run. I used this mainly along the back sides of the hidden runs. The front sides need to be accessible from below for cleaning and rerailing, so I needed something just high enough to catch derailed rolling stock, but not so high it'd block my hand coming up from below to clean the track.
Under Rico, I experimented with more of the chair stuff, screwed into the 1/2" plywood subroadbed - it actually seems to work ok. It's flexible, but should be plenty strong to prevent derailed stock from hitting the floor. In other areas, I ended up using very thin strips of masonite, again just screwed into the plywood subroaded. I found that as a little as 1/2" edge above the top of the plywood seems enough to prevent crashes - it just has to catch the trucks of derailed stock. At this low height, it's pretty easy to reach over it (from below) to clean the track.
And then, finally, I've started on the areas above the hidden track. The first is the upper-level connection between Ridgway and Durango, going thru the stairs. I got part of the subroadbed for this stretch installed. Below are some photos of the lower and upper tracks going thru the stairs, during install of the upper track. I created the upper track roadbed and laid cork on that, then installed it.
These two photos so the upper level track emerging from the stairs and entering Ridgway. Anchoring the upper level roadbed as it emerged proved a little tricky, as the lower-level track is right below it.