If you are going to use 4 cables, one in each corner, I would recommend 1 x 6's for the outside frame members (especially the 10' long members). 1 x 4's would be okay at first, but over time your layout would most likely start sagging in the middle, even if you don't have a lot of weight on it.
I have a 6' x 16' train table, with 1 x 4's (and a couple of 2 x 4's at plywood joints) spaced at 2' on center. I have 8 legs total, so I didn't have to worry about the framework sagging. However, the 1/2" plywood I used sags very slightly (probably no more than 1/8' to 3/16" max.) between the 2' spaces. I figured this might be a distinct possibility when I built the table, but decided at that time I could live with it, if it did. And so far, I'm okay with it. It's not a problem for me. But naturally, if I had wanted more guaranteed flatness, I would have went with 16" on center spacing instead.
