I have never been a fan of cork roadbed - too thin and ineffective at dampening noise. I like Vinylbed and its successor, Flexxbed. It is a rubber-like material. I use 3/8" thick stuff for the mainline and 1/4" for yards. It can be held down with brads (small nails) or glued. It's quiet. Covered with ballast, it looks great.
You can see some of the roadbed in the foreground of this photo.
Here's some ballasted Vinylbed.
A buddy of mine used cork on his old layout (I'm using his benchwork technique). It was loud in his train room because the cork really does very little to cut the noise. You should think about this. It's not cheap, but it works.
George