Hi Pilou,

> In fact I suppose that this rail scaling can be
> considered as a sort of third rail?

Yes, kind of... It works though as a modifier to the sweep, the sweep will initially be formed with the regular rails you give (either one or two rails), and then if you set a scaling rail, it will modify all the generated profiles of the sweep to stretch out in the way that I showed in that previous illustration.

The "main rails" of the sweep control the placement of each cross-section more directly - the "scaling rail" just controls a stretching factor.

- Michael