What Fred suggested is fine for a yard, if you want a suggestion of roadbed height but want to make the secondary trackage lower than the main, you can still use the cork he suggested, cut to the appropriate width strips. Shoulders can be shaved. The HO cork can be shaved down over a foot or so distance to meet the height of the sheet cork. I've also used N scale cork, but when you align the two halves so their shoulders are even with the HO cork, you have a strip down the center with no cork. I just filled that in with spackle.
Shaving the cork, for me, has been a PITA. Perhaps others have suggestions on how to do it. Instead, I've often built up a "ramp" on top of the lower cork. I take a piece of plywood (1/2" or more) some wax paper and a can of expandable foam. If there is a curve I cut the plywood to match the curve. The plywood will be screwed down at each end of the ramp, and its bottom side covered with the wax paper. Do not attach the wax paper to the wood. When the wood fits properly, apply the foam, put the wood on top and screw it down. When you remove the wood the wax paper will stay on the foam, you can trim the edges and leave it. Use knife,rasps or surform tool to get rid of the excess foam which came out the sides. I did this first on a s curve dropping 1/4 from a main to a freight house siding over a distance of 2 feet, it was easier than anything else I could think of at the time.