Some tips on Birail Tool
* The profile curves and rail curves must touch and have positional continuity.
* You should ensure all of the profile curves have the same number of cv's.
* Every rail should have the same number of cv's.
* Press V to snap to vertex.
* If you don't have the same number of cv's on all the curves, you should rebuild the surface.
Anders Sundstedt
CG Artist
https://www.sundstedt.se