Begin by tracing your layout onto 4ft x 8ft quality plywood. Recommend 3/8" thickness for maximum flexibility. For elevation use 2" x 4" studs - check for straightness - and glue with ample carpenter glue. Vary height of studs keeping rule 2-3% incline. Overpasses should have 6 3/16" to 6.5" of clearance. Since a Lionel train set is considerably larger than an N or HO train set, I recommend using wood instead of stryofoam. In all likelihood you will have to walk on your train set table more than once and the wood will support your weight. Space studs out every 10" to 11" inches.