My experience says 42". However when I built layouts for people I would set up some test levels of modules for the client to get a feel of what height they might select.
TMI...My own layout varies depending where you stand. Our basement floor was poured with a 6" pitch over 70' from diagonal corners to favor the sump pump crock. So if I had a flat single level RR there would still be variation depending where one stood.
From the first level on my RR to the fifth level there is 35" of separation. The towns and yards are each at different levels and all other right of ways are on a ascending or descending grade.
The rail head on the fifth deck is even with my eve level at the highest point of the basement floor
What is kind of weird is when I walk with a train along my longest grade which is at >1% I do not sense the train to be climbing up that much because we are both going up, although at 1% I hardly "feel" a grade.
At the start of construction, midway along the basement floor I shot a 360 degree datum line and marked it at 32" intervals on the walls, columns and return duct. Then I mounted 2" X 4" studs from which all construction was mounted.