That's the method I like.
I got styrene strips about 1/8" wide and used latex based contact cement to "glue" it down on top of my "subroadbed". Then I "filled in" with Duram's Water Putty and sanded smooth when it was dry.
I put my roadbed on top of this, and then the track.
IMO, a very "robust" method. Mine's about 8 years old and no problems. For the roadbed and subroadbed I used "VinylBed"by Hobby Innovations.