**UPDATED May 24th**

What you guys should strive for before trying to go for a NURBS car, is understand first how the curves work, familiarize with the surface tools, make some tests along and generally get a feel for NURBS modelling before attempting a complex surface as this one.

That said, let me give you some pointers on NURBS modelling which might come in handy:

• Opossing Curves should always have the same spans.
  To change spans on a Curve, use the Rebuild Curve Action.
• Opossing Curves should always have the same direction.
  To change direction on a Curve, use the Reverse Direction Action in the Curves Menu.
• When picking try to follow either a clockwise, or a counter-clockwise pattern. Don´t go picking them on a random order, I just use curves surface direction for this.

The Birail tools are not meant to be used as flat surface generator, but instead are to be used to create surfaces in which you want to control the location of generated isoparms.
To use them effectively, you should keep the preceding points in mind, remember to have them connected, and know when to use the Enter key.

• Birail1 makes use of three curves (One profile and two rails). It creates a surface that pretty much starts at the profile, and finishes at the end of the rails. To use it: Start the tool, pick the profile, then pick the rails.
• Birail2 makes use of four curves (Two profiles and two rails). It creates a surface that sweeps from the first profile to the second along the rail ones. To use it: Start the tool, pick the two profiles, then pick the two rails.
• Birail3+ makes use of as much curves as you specify (Three+ profiles and two rails). It creates a surface that sweeps from the first profile through all the profiles you pick along the rail ones. To use it: Start the tool, pick the first profile, then the second till you´ve picked all your profiles, press ENTER, and then pick the two rails.

For the surfaces you guys are attempting to make, I´d really use the Boundary tool instead. Remember Nurbs are 4-sided patches, and as such 4-curves represent the ideal way to construct them. If you take those pointers -and the ones above- in mind you won´t have much trouble working with the Boundary tool.

Again, I stress, work first with some surfaces, twist them, play with directions, rebuild them, change spans .. before attempting more complex surfaces and all. Believe me, you don´t want to see stitching in badly constructed surfaces.

As per your image bernhard, I´d say that does not qualify for a Birail operation, you´ll only get a poorly constructed surface. Build a four sided one, and just Boundary it together.
To stitch them together is a whole other business, however it takes the same exact pointers in mind.

Hope this helps guys.