Michael,

earlier you said: "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." When I used this jumping around 3D rail curve, rail revolve produced a shape as if the curve was planar to begin with. To me it seems like the Z value where simply not used or discarted since in this particular case the curve (rail) was drawn in the XY plane with varying Z values, the profile was in YZ plane and the revolve axis was in Z. I didn't save my work and lost it (power failure) but if you need, I can recreate my process and results.

I understand my rail as exactly the same X and Y coordinates as if I projected this curve on the XY plane and this is why I interpreted the result of rail revolved as if it discarted or didn't use the Z values. I just wanted to understand what was happening. Maybe I should have ask why the command didn't fail, since the rail curve wasn't planer?

As suggested, I did a search on "Swung surface" and most papers I've found are way out of my league but I saw a description of the functionality that is basically the same as what you describe earlier. I even found that NURBS is much older then I would have thought since it was first implemented in Fortran.

Thanks Michael, I learned a few things,
Felix