I didn't invent the trick, it's been done by MANY others, but I've always wanted to try it myself - using a mirror to create the illusion of tracks extending through the backdrop. In this photo, I've placed a mirror behind the trees, against the backdrop, making it appear that the tracks run slightly uphill and around a bend, disappearing behind a hill on the layout.
More photos and how-to's on my blog at http://dmirhillcitysub.blogspot.com/