John
NKP68, that's exactly what I did. I matched the riser thickness to the cork roadbed and cut it. I used Elmers to hold it in place and, once dry, continued laying track.
I don't believe in doing things twice, so I don't glue my track down and then ballast. I used "T" pins to position the track and turnouts then ballasted. The process of gluing the ballast also glues the track in place. This is contrary to the way many modellers do it, but it works.
Your choice to put your yard track at grade follows prototype practice. There are two large yards in this area as well as one across the river. In each case, the yard leads aren't built up as mainline track would be.
Here's a view down a couple of sidings on the layout.
This line runs behind the depot and then transitions back up to the main.
Depending on how much track you have down, you might consider "HO" roadbed for your main lines, and "N" roadbed for the other trackwork. This will give you the elevation change you desire.
If you want to transition from the roadbed to what's below it, you can gently sand the cork and make a smooth transition without any other products.
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Thanks Mike!
I am assuming you use the woodland scenics grade riser just like a shim and let it run out.
Woodland Scenics offers risers that can be used for that. They come in different grades from 1% to 4%.
I used their N scale risers for just that purpose on my layout. My switching area is mounted on grade with my mainline on cork. Once the ballast and ground cover is applied, the risers disappear.
The turnout on the left, off the main, is mounted on the riser.
Another option would be wood shims. The same used for shimming doors. The angle may be too severe though.