see attached rhino file.
dead simple sweep fails.
sweeping an arc (cut circle) along a straight line with slight rise in its route toward rear.
select black line (rail ) then select the arcs starting at one end and working to rear (the slightly higher end)
preview shows nothing, shaded is on.
ok and see nothing.
If I select the front only profile it works, giving three separate surfaces.
Sweep1rail fail.3dm (26.7 KB)