Hey that's very interesting OSTexo, thanks. I take it the rails are in the vertical plane when viewed from Front view? And the profiles are ellipses perpendicular to the rails?

Sort of on the same subject I came across MajikMike's MACH16 tutorial for a Networked organic shape. I tried it tonight and it gives great control as the control points can be manipulated, with the Networked solid following your changes - very cool. Only problem is getting the ends of the model smooth, as Networking used in this way is susceptible to non-tangent control points. I take it a two-rail system is not so sensitive at the tips?

Here's the tut if you have not seen it: http://moi3d.com/forum/messages.php?webtag=MOI&msg=5188.1

Thanks & cheers.