Joe, there are a lot of problems with the kind of table I think you are considering. It's hard to reach the middle without reaching far. An oval usually has one or two sides against a wall and this makes it worse. I have built over 30 train tables, but not one specifically for Thomas. I would build a loop on one end, a one foot wide section along your wall, and a loop or railroad yard at the other end. You can go around a corner if you can talk your wife out of 2 walls.
You can make this now so that it is on brackets holding it to the wall and high enough for your son. As he grows, you can raise the table every year or 2. Eventually, you will get it high enough to really make use of the storage space under the train table, which will make both your son and wife happy. But with all the room and window configurations that are possible, it isn't really feasible to send a "plan" for this. At this stage, I would put support brackets on every stud and put 3/8" plywood with 1x2 edges screwed to the supports. You may have to add more supports and/or legs under the loop and yard sections. A 1x4, with a lag screw on the bottom for exact length adjustment, makes a good leg and not much of a loss when you go higher. Putting 2 1x4's together in an L shape is even better.