The one I have seen isn’t quite that long, but it is set up so the plate (made of plywood) with the tracks rests on top of another sheet of plywood, which has a frame under it to keep it from sagging, just like the rest of the layout. This is attached to a modular layout taken to shows.
I’m thinking of one, but not quite that long.
Good luck,
Richard
If it’s going to be 13’ long, I would suggest that it be built like a turntable, with the pivot in the middle and the ends supported on some kind of rollers. That means only 6-1/2 feet between supporting points, which should moderate the tendency to sag. Of course I’m not suggesting a turntable that turns 180 degrees. Just one that turns enough to change tracks. Sort of a two-ended sector plate. The pivot could be a bicycle wheel hub or something similar.
Tom