With the tool extrude you first pick the profile curve, second the railcurve, to create the surface.

It´s important to use within the extrude tool the option tube!

6.) Rebuild the railcurve
 

To get an uniform profile you have to rebuild the railcurve to 12-16 spans. It´s important to get in a regular distance a "support curve". Same principe as the revolve.

7.) Positioning of construction planes

A second way to position a construction plane is to snap it on a plane. You create at the end of the extrude a surface with planar surface. To position the construction plane in the center of the surface, you have to increase the patch precision to 4 patches. Press the CMD button and snap at the intersection point.

So the construction plane is in the same area as the surface.

8.) Positioning of the luminary