Thanks to you both. I am researching and hope to come to a design decision. I had been thinking of a shelf and am glad you commented on visibility, I was wondering about that myself. The closer to the ceiling and smaller the room, the angle of view could be such that if you were seated, you wouldn't even see the train from some angles! I wanted 2 parallel tracks, but thought I might not be able to see the inside train at all from some angles.
My first idea was wood suspended by metal rods, but that now seems too much trouble. I will probably use a ledger board and brackets. The shelf will be about a foot from the ceiling and run in a large oval running through openings between my kitchen and the formal living room. The kitchen has a breakfast nook with a bay window, so that will be the focal point. But as it runs through the dividing wall with a large opening to the living room, there is nice seating there too and it is where we put the christmas tree every year.