I'd like to point out that baltic birch plywood and birch veneer plywood are two different things. For our purposes, the veneer of the plywood is immaterial. Unless you are planning to clear coat it and see it, it will be covered by scenery of some kind- paint at the minimum. What is important for quality is the core, and Baltic Birch is generally manufactured to higher standards with more plies and fewer voids. But for train tables, almost anything will suffice, honestly.
If you plan on walking on it, you are making a deck. You need more cross members, more legs and thicker ply. If you are not planning on walking on it, you are making a table. My small layout is a table and it is 1x 4 framing with 1/4" ply. so I can put it away after Christmas. I have a 1" layer of foam on this, so it is as flat and rigid as it can be.
It's definitely possible to over-engineer a train table. The HO guys use L girder construction- very light and flexible for design, but that is a whole different discussion.
