The "extended rail" type thing would be something like this - the blend would be synchronized to each side like you've got here:



But instead of it actually stopping there, the blend would continue on with the shorter surface being implicitly extended so it would form an extended blend piece like this:



Then the extended pieces are intersected and the blend would be trimmed to form the trailing edge of the blend, like this:



I guess that seems more likely to work than trying to directly make a single surface cross section in the sharp corner...

That seems pretty tricky to attempt in an automated way, and I'll need to look at various other discontinuity cases to see how generally feasible it is... Could be something to try in v4. Probably until then a tight bend in the wing surface trailing edge rather than an actual discontinuity would probably work with Planar sections.

- Michael