On the train tables, I modeled mine after the ones Glazz74 made for his truly impressive display. They are 3/8" thick plywood with 1x3 frames to give them stiffness screwed around the edges and across every 2 feet, so you use two 8 foot 1x3 pieces and five 4 foot pieces. The concern he had was trying to get the overall weight down so one person could move a 8x4 section. They weigh about 40 pounds each, which is less than a single sheet of 3/4 inch plywood.
As for legs, I'm using 2 inch PVC pipe. Four legs for each table, so I can put them up and take them down without a lot of effort or needing a bunch of sawhorses.
The legs are pretty clever, you glue 3/4 inch plywood pieces 6 to 8 inches square in the corners and mount a 2 inch end-cap via screws in the center of the plywood. Then from a 10 foot two inch pipe you can cut four 30 inch legs. Sand the end of the pipe so it goes into the end-cap smoothly - you want to be able to remove them easily.
I didn't make it to the Puyallup train show last weekend, but last year Glazz74 had made a cart with wheels for moving and storing his PVC legs. His setup is VERY impressive - I think it's about 36 feet by 16 feet!!