I'll be raising up some of the industrial areas on a layer of sheet cork, in order to raise some areas of the terrain very slightly, and to level it off with the tracks in places like the scrapyard and team track.
Industrial trackage isn't usually as uniformly nicely graded as a well-kept mainline, which is the effect that using the cork roadbed kind of gets you. By raising certain areas, profiling the spurs down in others and leaving some areas down to bare plywood the terrain will have some variation. It won't be extremely deep below track level, but that's ok. There will be a bit of a drop below track level in some places thanks to the roadbed, and some very small hills and rises can be introduced in others.
Ultimately, most of the layout will be taken up by industries and parking/driving areas, so it will be flat, but not billiard-table flat.