Where my track has roadbed under it, I use cork. Where I want to run an industrial siding off the main line, I lay the cork partway into the siding too: if you have room, the farther the better. I glue it in place using yellow carpenters' glue, then use 1 1/2" nails, tapped in just far enough to bite into the plywood subroadbed, to hold it in place until the glue dries. (overnight) After removing the nails, I use very coarse sandpaper (#36 garnet), wrapped around a small block of wood, to taper the cork from full thickness just beyond the end of the diverging rails of the turnout, down to nothing at the end. The reason for laying cork as far into the siding as possible is to keep the transition from the mainline down to "ground level" as gradual as possible. The sanding, even if the transition is 2' or 3' long, takes only a few minutes to do. Save the sanding residue, as it's useful as scenic fill elsewhere on the layout. If you don't have a lot of room for the transition, as long as the grade is not so abrupt as to cause cars to uncouple, you should be alright.
In the photo below, the double main lines are on cork, while the industrial trackage, both near the lumber piles and at the rear, between the station and the icehouse, are all right on the plywood subroadbed. The transition down begins just past the guardrails on the turnout near the telegraph pole, and ends just before the guardrails of the turnout which is near the road crossing. In other words, the turnout off the mainline is completely at the same level as the mainline, and the turnout near the road crossing is completely at the same level as the subroadbed. The transition between the two is accomplished in a stretch of track about 15" long.
On my layout, I extend the mainline ballast out onto the diverging route of the turnout, gradually increasing the proportion of cinder sub-fill as the track gets farther away from the main. I also begin to mix in some earth-coloured ground foam, different colours and textures of ballast, and, of course, some ground foam weeds. I like to use very fine green or yellow foam to begin a subtle colour transition to areas with heavier concentrations of "weeds". On an industrial siding, the ballast might be almost non-existant, with most of the ties covered with "dirt", and generous amounts of weeds growing between them.
Wayne