Hi Felix - rail revolve works by taking the regular revolve and basically applying a scale factor for control points of the rail by how far they are away from the revolve axis.

But it basically expects for the rail to be planar and perpendicular to the revolve axis, not to have the rail jumping around in 3D like that.

For something like that you'll need to use an alternate surfacing method, like Mike mentions with Sweep.

In the future I want to expand Rail revolve to have an additional mode that will work more like sweep instead of the scale factor technique. The nice thing about the scale factor technique though is that it is exact - the result surface has the exact same control point structure as the rail curve, while sweeping method use a kind of iterative fitting process to generate the surface.

- Michael