YO FLINN!
We don't hear enough from you!
Biggerhammer is on right track.
Don't even attempt this kind of project unless the job is going to look "Finished". That's EVERY wife's primary complaint with these things, and it should be avoided at all costs.
You're talking about exposing what many consider an "odd-ball hobby" to everyone who enters your home. Be prepared to have your feelings trashed when you get more compliments like "Nice finish on that molding.", instead of "WOW! I love that observation car!" Reactions will be far more mixed than you ever recieved taking people into a layout room, and your wife will be quizzed by the ladies more often than she cares to be.
Of the few I have seen, the ones I liked had a wooden valance to conceal lighting from above, while others had soft reddish lighting to simulate sunset from behind, making the train a ghostly sillhouette as it glided by.
Unless you have seating, such as a sofa, far enough back, putting the base within 14 inches of the ceiling can be a waste.
Another consideration is maintenance. How's your back? I know how bad mine is. High dusting needs to be done a bit more than the low stuff.
Derailments are my main concern on a project like this. The molding from the bottom needs to come up far enough to prevent tumbling, but nicely finished wood will block your view of the train. Why not consider a shelf and enclosing it with plexiglass? It would significantly reduce the dust factor, but must be easily removed to cure a derailment in the event of one.
If you're considering it in a room, have a seat and see if you have to crane your neck to see where you want to put it. If that's the case, abandon ship.
If going with what Dave Flinn called a "normal height", I take that as four feet. What a nice diorama that would make going through a book case, and enclosed with plexiglass to keep LITTLE FINGERS at bay! Restaurants and pubs do this quite often.
For merely extending a run, if John Taranto is out there, he had an extended run through his laundry room where he encountered some problems. If you're out there John, dive in, please!
Interesting topic.
George.