I have a 6' by 16' table with 6 legs, 4"x4" built over 20 years ago. 2"x6" between the legs with smaller bracing for the plywood top. The top is 3/4" plywood. I can stand on it without any problems. I can screw into it to hold things in place.
I have built two layers on top of the table without issues. I run standard gauge on the bottom and O gauge on the upper 2 levels. 1/2'" plywood won't cut it for standard gauge. Plywood sags as the engines run over it. Experienced by other members of the group (D3R). When I built the table, I did not know I would get into Standard Gauge.
Upper levels are O gauge only and use 1/2" plywood without framework and supporting posts about every 12 inches. No problems in 10 years.
1/4" plywood is not good for a two level. 1/2" is marginal with upper level. 3/4" is good for upper level support. It will also support your weight. You might be able to save enough on framing to pay for the 3/4" vs 1/2" plywood.
