One of the reasons that I prefer cork roadbed is that it's easy to make a smooth transition down to the sub-roadbed. Simply lay the cork as you normally would, ending it where you want the lower area to begin (not where you want the slope to start downward). I use yellow carpenters glue to secure the cork to the plywood sub-roadbed, temporarily tacking it in place with partially driven 1 1/2" nails. When the glue has dried, the nails are removed. Now, wrap a piece of coarse sandpaper around a block of wood, and start sanding. I use #36 paper, as I have tons of it on hand, but any really coarse type will work. Try to make the slope as long as you can, especially if the track is part of the mainline. For industrial sidings, the distance can be much shorter. Don't be put off by the idea of all this sanding, as the cork is very easy to sand and shouldn't take more than a few minutes. Save the residue, too, as it's useful as fill or ballast.
In this photo, the diesels are running on track that's on cork roadbed, while that turnout at the top of the frame is sitting right on the plywood tabletop. the transition is about 14" long.
Here's the same area without a train in the way. The turnout for the coaling tower hopper track, hidden behind the roof in the foreground (but visible in the photo above) is on the plywood base, while the dumpshed is on full-thickness cork, with the transition using up only about 8".
While it doesn't show very well, the mainlines in the centre of the picture curve around the backdrop, in the distance, and make the transition from full roadbed, starting from about the far end of the second boxcar, down to plywood around the corner, in about 3' of track.
Wayne