If I am reading your question correctly, you are asking how much overhang would be possible with sheets of plywood on top on your existing 5'x10' table. Assuming the table is strong enough to support the weight, I think you could secure three sheets of 3/4" plywood across your table, creating an 8'x12' tabletop. The overhang on the ends and sides would only be 12" and 18" respectively, which should be no problem considering the stiffness of 3/4" plywood. I do think I would attach a length of 1x3 underneath the edges of both sides as a stiffener, and also to keep all three sheets from warping. A bit of overhang is good, it will give you some "toe room" when standing next to the table.
Standard gauge track is properly described by the diameter of the circle that would be made by by the sections. Just like 0-72 means 72" diameter of 0 gauge (1.25" gauge) track, STD72 track refers to 72" diameter standard gauge (2.125" gauge) track. Both diameters are nominal, i.e. not exactly the measured dimension of the circle, just like a 2x4 piece of wood is actually 1.5"x3.5". In the case of STD72 track, the actual centerline diameter is 69.750.
I you want to have a double track, I would suggest STD57 and STD72, or STD72 and STD87 loops. The resulting 7.25" track centers are ideal, and trains prefer curves wider than STD42.
Kirk Lindvig, USA Track LLC
