Hi Don, it's often times difficult to give very good feedback or answer questions very well just looking at screenshots instead of the actual model.

For example in your case there I would want to zoom in, look at things from several angles, turn on control points to see what those looked like, examine the control point structure of your original curves, etc... - those are all things that I can't really do just by looking at a screenshot.

So if you could please post the 3DM model file along with your question, it is a really big help.

A little lump like that in the middle of your shape could mean that there is a little wiggle or slight break in tangency in your rail curve right in that area.

Or possibly it could be a bug in MoI that should be fixed up - if I have the model file that lets me examine that possibility a lot better as well. It could be possible that it would be better for MoI to build a sweep in that situation in multiple pieces that are joined together rather than as one big long single surface piece. If that's the case then you could do the same thing currently by using Edit>Separate on your rail curve and building it in one section at a time (use the ending edge curve from the last section as the profile for the next section) to get the same kind of construction.

- Michael