Cork sheets are perfect as track bed underlay and these sheets can be used to be cut to the exact shape and size you need.
Cork is used under tracks for a couple of reasons - Mainly it dulls the noise that the trains make when running on rails mounted to wooden boards (so definitely use if your layout is in the house!) - The other reason is that it nicely raises the rails and makes it look all the more realistic once ballast is added.
The sheet is 3mm thick and is 24 inches (610mm) x 36 inches (915mm) and is rolled up for you to use however you wish.