Do you mean ceiling height above the doorways?
The interests is out there,you just about described it all. It is basciallly a display. A wider shelf will get you two lanes with switches. With wide curves you can run a long train.
Look at the dmiosk
thread. He converted his cellar.
Design it as a shelf or a suspension bridge from the ceiling. The side trusses give you some protection from a fall. Speed has to be controlled. I am not a big fan of ceiling trains in a childs room. Forgive the corny joke.
For Kids, a piece of board under the bed is better.
Now the walk around style is the Bees Knees, with depth you can go all out. That is what you find in the train mags.
I placed a link to the Thread For Dmoisuk
I know bad spelling.