Hey guys, I appreciate these great responses. Santafewillie, some good points, I might go with your heights, and crawling under to the access places won't be a problem, my waist is 32" and I used to explore caves! The only time I need to access those is laying track and building scenery, not when operating unless there is a derailment.. What is the depth of your upper layout shelf? Also, do you have a valance above your upper layout that contains lighting?
Topherisme, I like the idea of a swinging gate, hinged like a door and maybe a roller wheel at outer end of gate to help hold weight, just might be a good thing. Gate could possibly be built with shelves to hold rolling stock, and framework that the gate is attached to could be securely fasten to floor at bottom and benchwork at top. I am still considering doing away with the Wye altogether as suggested by PowerCab, which would make human movement easier, and make more use of the turntable.
Logansawman, My old N scale layout didn't have a staging area but I want one with this layout, the small size of the room challenged me to think outside the box. I see myself sitting on a stool when doing staging, might have shelves underneath staging to hold rolling stock, although I still have legs that need someplace to be!. I don't think I will go high with layout and I will only be underneath when wiring, whereas most of time will be spent upright laying track, building scenery, and running trains, and the depth of the layout means I don't have to go under very far. Considering I change my own oil in my pickup and go under my camper trailer for maintenance, I should be able to handle going under the layout, however now the 36" height makes more sense since I measured myself sitting on my butt and I would fit fine with that 36" height, 32" would mean laying on my back, or having surgery to remove a few vertebrae!!
Responses give me many things to consider...Thanks!