Many, many years ago, I created a layout using Gargraves sectional track with plastic ties. (I just recently sold some of the remaining unused boxed track in my York Virtual Bandit Meet thread. Just one more box left!!!)
The stringers run parallel/concentric to the rails (but slightly offset), and as I recall were flush to the bottom of the ties. The clips are essentially an inverted U-shape with a tab that can be screwed into the roadbed. At the time, I used cork roadbed atop a Homasote/Plywood sub-roadbed. The inverted U-shape portion of the clips wraps around the stringer quite nicely, holding the track firmly -- but not too tightly -- to the roadbed.
I really liked the results. The fact that the screws didn't actually go through the ties themselves resulted in a slightly quieter overall running of trains -- at least as quiet as could be expected with tinplate rails (as opposed to solid-rail track systems). If you plan to ballast the track, you won't even see the stringers and retaining clips when all is complete.
Hope that helps.
David
