On the South end I had a door to work around.
It was only 35" from the facing to to wall.

35" became the width of the frame and 37 ½" the width of the plywood top because of it.

1st, we started with the North end of the room.

  We constructed a 7' x 35" frame of 1x6's
and mounted it to the wall.
(We made the end pieces 37 ½".) 

Why 7' long? (It was a portion of a frame I had previously used in my old home.)

Note the 3/4" holes drilled in the frame to allow routing of wiring later.

Also, the braces make neat places to hold extra 3' sections of O track.

Note the LEVEL we used to make sure the table was "in the bubble". Using the stud finder my dad got me for Christmas, we mounted the frame to the wall studs with a few drywall screws. After all of the table frames we found to be "in the bubble", we used 1/4" x 3" lag bolts to secure it to the wall.

Another shot of the North frame showing it's one 2x4 leg.

A vent we had to work around on the West wall..

Because of the vent, 
we decided to make the frame 40" high.

2nd, we mounted the South end another 7' x 35" frame of 1x6's.  

3rd, we measured the distance between the 2 frames. 
We then built and installed a 35" W x 37" L box between them. 
This is an underside shot. 
You'll note we used 2x4's  to make it easy to screw to at the joints where the 3 pieces of plywood meet. (We also designed the box so that I could later extend the vent to the edge of the table if the need arises.) 

4th, we ripped an 8' sheet of ¾" plywood 37 ½" wide for the South end. 

(Note the 2 ½" x ¾" notch we cut to clear the door facing.)

5th, we ripped another 8' sheet of ¾"  plywood to 37 ½" wide and installed it on the North end.

6th, we ripped an scrap sheet of ¾" plywood to 37 ½" x 12 ¾" to fill the gap between the other 2 pieces.

Here's a view of the table we finished in one afternoon. 

We used 1 ¼" drywall screws to secure the plywood to the frame.

I have some green felt to cut and put down before the track goes down.

Another view.

It's easy to reach across for me and my Lionel stool will fit under it when not in use.

(I cut the legs 38 ½", so that I could lay down a 2x4 to distribute the weight. But, I don't think I needed to as the wall seems to bear most of it.)

An underside shot of the table.

(I want to do an "Old-West" theme using my Fort Apache set and other Cowboys and Indians I had in the 50's / 60's.)