My 2 cents regarding layout height... One portion of my layout is at about mid-chest (around the top of the strike zone). This shortens my useful reach, but it also makes it easy to sit on a rolling stool underneath to work on wiring and gives a more interesting viewing perspective. It might also be a bit high if being viewable by small children was a consideration. The other portion is about waist high - easier to work on from above, but I need to work on my back on a creeper to do things underneath, which I don't enjoy.
When I do it over, I'm not sure which way I'll go.
It's a personal decision based on reach, mobility, and preferred viewing angle. Perhaps it would be useful to set up a table with some trains on it and raise/lower the height and see what works best for you.