Here is the honest answer and the best free workflow to generate professional railings without spending a dime. (by Fredo6) Wait, isn't Curviloft for organic shapes? Yes, but its "Extrude Edges" feature is a hidden gem for railings.
It allows you to select a path line (your stair slope) and a profile face (your handrail shape) and extrude them instantly.
I have written it in a helpful, "tutorial-style" tone, assuming the user is frustrated with manual railing modeling. How to Build Complex Railings in SketchUp for FREE (No Expensive Plugins Needed)