I pulled the tunnel off the layout (hopefully for the last time), to add the background in the tunnel. Looks pretty good IMHO. Since it's a gravel road with grass surrounding, I added some ground foam to the gravel on either side of the tunnel. I will continue this for part of the road and transition to just gravel since it runs parallel to my main line. I put some plaster cloth down on the board too.
I saved this piece of ground cover when I pulled up the grade on the other side of the layout. Looked like a perfect spot to use it.