As you can imagine, I've been reading and watching a lot of "how to" articles for model train hobbyists, and a couple have recommended getting two 4x8 plywood sheets from the lumber yard and ripping one of them to make 1x3's for ribs and skirt... but after walking through Home Depot, I really have no idea why they would recommend that way, as plywood compared to 1x3's are very expensive. In fact, my calculations say that you could piece together 1x3's to make a 4x8 sheet for about half the cost of a 4x8 sheet of plywood. Oh well..
Anyway, as you can see, the table looks pretty good, ready to build the foam base on top. I purchased 2'x8' sheets of 1/2" construction foam to build the base and scenery (including later hills and tunnels), as this is a pretty clean and easy material to work with and recommended by nearly everybody for this kind of application.
