How to create this???

 From:  Michael Gibson
3550.5 In reply to 3550.3 
Also for "preserve tangent" mode for the 2-rail sweep to work, the profiles need to not only be on parallel planes, but also have a shared tangent direction between them where they touch at least one of the rails.

So you want to have things like horizontal and vertical curve tangents in the spots the profile pieces touch the rails.

- Michael