In the attached file shows how I did it, not saying its the right way, just 'appears' to work for me with rail sweep, if this is what you are after.
The blue arrows show the + z axes direction I'd expect the profiles to follow, two profiles required because going off your earlier picture, you mirrored the paths. I angled the profile to try and compensate for some twist, before applying place on workplane to set the z axes. The extra 'dimple' I added to the profiles was just so I could see better when the rail sweep was roughly correct.
I've no idea if this helps or not. I totally agree with Henry. Sweep is often much easier to set up than Rail Sweep.