My railroad is all indoors and this approach might be a bit too much effort and expense for your use.
My benchwork is a sort of modified L-girder and the base of the roadbed is laminated Masonite hardboard splines with Flexxbed glued to it with vinyl coving adhesive. The SVRR track is then nailed through the nailing holes just enough to secure it. There will be ballast of some kind eventually once I start doing scenery.
I’m kind of ambivalent about Flexxbed. It works okay, but I guess that I was expecting it to be quieter considering the density of the splines and the minimal “coupling” of the track through the Flexxbed. It was also a bit difficult to smoothly bend on the 8’ diameter curves and to hold in place until the adhesive set; needed a lot of clamps and weights.
I decided to stay away from cork as, when I used to do HO in my younger days, it would tend to dry out and get a bit crumbly. I used Atlas back then, perhaps the binders that held it together have since improved.