Single symmetrical Surface Through Half Sections And Rails

Hello! I would like to obtain a symmetrical single surface with seamless junction in the middle from the blue cross sections and green rails as Inputs (half sections, degree 3 with tangency in the junction) and apply symmetry at some point. The question is when. I would like to operate a “loft with rails”, or a “sweep3”. It is really important for me to stay perfectly close to the rails (outlines apex), avoiding having a polysurface or more control points. I need advises and help about the logic and order of construction. How should I tackle the problem ? I can’t merge a simple symmetry into a single surface. Is it a question of degree (curves or surface) ? Thank you very much

Single_Surface_Through_Half_Sections_And_Rails.3dm (373.7 KB)

The primary reason you are having problems getting a single surface that is smooth is that your curves aren’t tangent. You can use Gcon command to check that.

I would make this using loft, like this:

Single_Srf.3dm (224.3 KB)