Hi Bruno, can you please send me the .3dm file that has the fillets missing in MoI but visible in Rhino? I'll take a look and see what's going on there. You can e-mail it to me at moi@moi3d.com if you want to keep it private.

Is it possible that they are block instances or mesh objects instead of regular NURBS objects?

re: Planar rails - it's when they are co-planar that 2 rail sweep has a very well defined frame normal. Even if they are each separately planar but on different planes it's a more difficult type of situation. Imagine that the sweep is formed by connecting a line between each of the rails. If this line starts to slant diagonally in some way as one rail goes around a bend while its associated point on the other rail is still in a straight area that can be bad.

- Michael