Hi Pete - the corner mitering that usually happens with 1-rail sweeps is mostly for 2D curves.

When you have 3D curves bending around all over the place, they don't easily intersect with one another in the same clean way as a 2D curve path does - meaning that the pieces don't just intersect with a plane.

Also this particular kind of crossing tube intersection happens to be an area that the surface/surface intersector in the geometry library that MoI uses has some difficulty with.

So those are some of the reasons why you're having problems there, it happens to be a problematic area, not that you're actually doing something wrong.

- Michael