Ralph,
I use Homasote for roadbed. I had a mill shop cut the 4' by 8' sheet into 2 1/4" by 4' strips. For curves, I make cuts to within 5/8" of the opposite side with my jigsaw at a 45 degree angle every 1" to allow the Homasote to curve. Then I nail it down with 1" paneling nails. Then I sand the surface lightly with coarse emory cloth on a sanding block. Time to vacuum the dust. I also paint the Homasote a brown to resemble earth. After it dries, I sand it again to remove any roughness created by the paint. Time to vacuum again. I lay the track and secure it with #19 by 5/8" brads through the holes in the ties. The brad will disappear into the tie. On occasion, it will not. Then I use a nail set to gently nudge it down. A #54 drill bit will make additional holes which are needed to lay the curves.
Hope this helps.
Ed