You will need to make the "inner rail". It will be at the elevation that you want the top of your surface to finish at and in the middle of your original rail, it will be oval- like in shape. The sweep2 command needs to have 2 separate curves to guide Rhino in the making of the surface. In this case they will both be closed cuvres. The small area that is left in the middle can be closed using the patch command.

I must not fear.
Fear is the mind killer.
Fear is the little death that brings total obliteration.
I will face my fear.
I will permit it to pass over me and through me.
And when it has gone past, I will turn the inner eye to see its path.
When the fear has gone there will be nothing.
Only I will remain. 

 Frank Herbert, author of Dune