Good start with base color - now use other earth colors and patchy areas of a second coat of that color. may want to wait until you get a track plan.
Do you have any half-straights and half-curves in O27?
Oh, no one answered your question about the Midwest cork - if it is in your budget - use it. You will have a very quiet layout and make the experience more enjoyable. Painting the cork with a single coat of gray will look like ballast.