This is not possible to do by default because Railing Posts are always placed centered in Railing Reference Line Nodes. Additionally, no 'Railing Post' GDL Object offers an offset parameter so the Post could be offset along the Reference Line. GDL Objects could probably be modified to provide such parameters.
However, let us think about how we could find a way to solve this right now.
My idea is to use the "Profiled Post 21" GDL Object for Railing Posts. It allows you to use Profiles defined in the Profile Manager for your Posts.
So all you need to do is to define Post Profiles where the origin of the Post is on its left side, or on its right side, for the starting and end post, respectively, or vice versa. I think this way you could make it work.
There is one additional thing you may have to do: by default more than one Posts may be selected when you click to select any of the Posts. If you right-click and use the "Create Independent Segments and Nodes" command of the context menu, each Railing Post can then be individually modified to use the proper Profile.
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