I'm just getting started in model railroading and have heard/seen several suggestions about different surfaces (Homalite (?); soundboard; plywood; extruded foam; ????) and was looking for pro's and con's of your experiences. Any feedback is helpful.
The major percentage of my benchwork is surfaced with - air.
The reason is that I'm modeling an area where O-Kamisama scrunched up the surface and stood it on edge. Think West Virginia (or the depths of the Grand Canyon.) The only flat spots are at stations or (sometimes) under buildings. All the 'between stations' track is on grades.
So, what supports tracks and buildings? Plywood 'cookie-cut' subroadbed (raised above L-girder benchwork on risers) covered with a layer of fan-fold underlayment (foam about 9mm thick) carved to ballast contours, then a layer of cardstock (full size track template or road/building foundations.) Adhesive of choice is latex caulk.
Landforms are attached in various ways to the edges of the plywood and supported where necessary on additional risers. Basic structure is cardboard strips covered with plaster-soaked paper towels (aka hardshell) Details are added with whatever material seems likely to give the best results. Parts of the terrain are designed to be lifted off, thereby providing access to the hidden tracks below.
I have also used steel studs, laid like rain gutters, to carry hidden tangents through the depths of the netherworld. A layer of fan-fold underlayment, a track template and three layers of latex caulk have proved to have excellent sound-deadening qualities, even for my ancient, metal geared locos.
Chuck (Modeling Central Japan in September, 1964)