Rooster and Rick make some very good points
I’ve used ladder construction and Freddy’s method, and I have a friend who uses flat exterior grade plywood tabletops. I’ve just disassembled my pike in preparation for a move. Outdoors I’ll use Freddy’s boards on a frame, but inside at the termini/intense switching and yards I might go with the flat tabletops. These give the ready option of altering the track layout as often as I like. Not that I’m likely to change it much or often, but at the beginning I really like to be able fool around with my options. Ladders are ok for mainline runs among the tulips but they really don’t allow for much flexibility, do they?
All the same, if you’re going to make a wooden ladder, you’ll want to be able to flex your material. I do this for curved trestles, making my stringers of three laminated strips of cedar secured together with 1" drywall screws. I soak them first as follows:
My SECRET TRICK for bending those thin cedar strips: I use a length of black pipe. I plug one end, fill it with water, drop the strips in and stand it all up vertically. I take 'em out when they’re well soaked and flexible. Then I form them up on the bench, where I have drawn the curve and driven finishing nails along that curve to hold the strips. I screw the strips together with 1" screws alternating inside and outside the curve.
I have had a serious problem here with slaters, or sow bugs, also known as St Anthony’s pigs. They love to eat wood, although they seem to leave cedar more or less alone. I’ve also had to deal with paper wasps. They eat wood for the pulp to build their beautiful nests. Finally, I’ve had to deal with carpenter ants. As a result of these challenges, I’ve become a confirmed user of wood preservative, especially on non-cedar woods out there. Cut ends of boards are particularly vulnerable, so if you have similar problems, be sure to coat them.