What smoothness? Even without the rails the moldings that go in place of the rails on a "railless" car don't make it smooth. Just semi-flush. You would have to actually remove the moldings, fill in with fiberglass or body filler or something and then smooth that over and then raise it up to the body side by the door so you had a "smooth clean line" across the top and then have it the top re-painted to match. To me it always looked like the moldings were just there to fill in the area because it was missing something. Once I found out about the rails it seemed like that was the intention but never done in mass, just on the "rare cars".