I'm working on a stretch of shelf railroad where I have about 5" of space between the track and the backdrop. As is comonly done, I painted a tree line on the backdrop, then added lots of just trunks. Then I planteda row of trees, Woodland Scenic armatures with all teh branched bent front and side, so they could be placed against the backdrop. Smaller trees and some brush are placed in front of these to provide some hiding of the backdrop between trees. It is just about impossible (for me anyway) to achieve the intermingling of branches you see in real life, and hard to get the trees very close together as a result. So the smaller trees and brush are needed in such a narrow space. However, it seemed that I needed some filler between the tops of the trees too, as they were spaced to regularly and appeared to sparse. Here is a photo of the trees just sitting in place with just two smaller trees placed in front, while I was trying to see what I could do.