My Marx layout has a track plan that is 27" x 54" Here's a shot of what the plan looks like:
In the upper left corner of the picture I used a Marx tin tunnel, and there's a Marx bridge that is likely close to the same size as Lionel's is in the front. There's a bit of room for a few small buildings and signals too:
Perhaps you could make the elevated line go along the backside and turn on one end toward the front (or maybe even both sides and make a U).
Hopefully this gives you an idea or two,
J White
