Hi,

some explanations (i'm french, forgive me for my poor english)



the basic shape. I use Max' tools (split, bridge, scale)



subdiv script




boolean substract with red curve




i join top edges to make a curve. I copy it and move the copy.



too many points. So i use Rebuild command




rail sweep (blue rails)



the result




iso curve (yellow). i reshape it....



... and make new rail sweep (two red profiles and two blue rails)



the basic shape (.3dm) is in attachement.