John,
I would suggest using plexiglass at the edges as the others have mentioned, or maybe a short hardboard sideboard with a skyline (background scenery) painted or printed on it. My small layout is set up the same way yours is, and my track is very close to the edge. If I move it in, the curve radius will be too tight for what I want to run on it. Therefore, I am planning to place a guard of some sort to prevent the trains from falling off, and am thinking about having a background photo or painting on it as suggested above.
Just my $.02!