Alan,
I am glad it worked for you. I wasn't sure if I miss understood the question or not.
I find as soon as I try to line up wall underneath a railing it is hit or miss (most often miss) if it works. I have defaulted to doing it this way for a while now.
Steps to create object. I used your plan (this way the stair height was already determined. I went and put that one railing on a different "Wall, Railing 2" layer. I then went to "All off" Layer set. Made sure everything was off, tweaked the rail post height from 36 to 37" so that the rail connected the post. At this point all I see in the 3d view is that one rail. (this is where I took the image of the railing that I posted. With only that railing showing I went and Tools, Symbol, Convert to Symbol option. Clicked advanced options, gave it a name and voila its in your library.
i4HomeDesign's post in Wall Under Stair Disappears When Railing Is Added. was marked as the answer