Here's how I model "buried" track, either overgrown industrial sidings or tracks buried in "concrete" (eg on a wharf).
My module benchwork is built out of balsa wood.
To bring the level of the terrain up to sleeper (tie) height, I first glue down a thin sheet of balsa (about the same thickness as the cork underlay). Then I scatter scenic material over the sleepers and the "raised" terrain surface. In this way, the tracks look like poorly drained, poorly maintained, half buried industrial trackage (centre and right).
For my wharf area (left) I simply use a thicker sheet of balsa topped with sheet styrene (painted light grey). The combined thickness of the balsa/styrene sheets should be just shy of the railhead for ease of cleaning and operation.
Might be of use to someone planning something similar.
Gavin Miller
Perth, W. Australia