I am glad I checked out this thread, lots of good ideas.
Only things I could think to add is to make sure the viewing access isn't pinched. 36" min aisle required for wheel chairs but that min width makes impossible for others to walk around/with someone in a wheel chair.
Also give some thought to what the edges of the layout will look like. A small imprvement such as maybe painted paneling instead of curtains around the edge of the layout can make a huge difference in the overall aesthitic. Once things get underway have some with no interest ( usually a significant other!!) come in stand by the entrance. Get their opinion about the look of the room before they close enough to see your details.
Oh - my 7 year old loves to find hidden scenes such as mining areas below the layout where most adults don't even bend over to look at.
