Well, first you will need to draw pa picture of what the top of your table will look like,
Next you look at the picture (in your mind’s eye) to see what sort of frame work you will need to support this table. You have seen the ads in MR, and so you know what the underside might look like. LION puts his cross members every 16" inches apart, that way him can get a power drill in between them if he wants to. Too short a distance: no power drill, Too long a distance and the top may sag. You know you need runners the length of both sides of the layout, and the cross members every 16". You will also need legs. How tall will your table be?
5’x10’ is marginal fro reaching the middle, but that is not the LION’S affair.
You have two 10’ long edges (East and West) you have two 5’ long edges (north and south) . At 16" centers you will need another 8 pieces 5’ long. But that is rather long, If I were doing that I’d make those members “L-girders”
A 10’ span is a little long for just four legs, yet more legs will only make for more instability, and if you build them for this place on the floor, and then try to move them to that place over there, you will find that the floor was not a precisely build as was your train table.
OK, say the table is 42" tall, you would need eight boards 42" tall and butted together in an “L” shape.
LION would plan to have cross members from the bottom of the outside legs to the middle of the 10" side just to give more strength and stability.
LION would use all 1x4s for everything, and so would figure on
2 pcs 10’ long
10 pcs 5’ long
8 pcs 42" long
4 pcs about 6’ long for bracing.
What are you going to put on the top of your table? Plywood? You will need maybe three sheets. (A 4’x8’ table would indeed be easier.
Here there are pictures of the