In the attached file I have two sweep paths (rounded corner rectangles). The one sweep path was created using offset so that. while the inner corner is tight. the different in radius is the same as the distance between the rails.
That looks like you’re confusing a faceted render mesh with the actual surface.
Switch to a Wireframe display.
Does that look better?
You can also kick up the density of your render mesh to see if that changes it.
File > Settings > Mesh
i dont think so John, there is clear indication that the surface is selfintersecting.
regarding the actual problem i am not sure i can help, usually adding slashes helps, but that does not bring anything on this model. it seems that the inner radius is far too tight for this to work. if sweep 1 works for you then there is no need to search further, if not you might have to find an alternative method of modeling.
Too large a file tolerance relative to the scale of the geometry results in errors or other problems. A smaller than needed tolerance can result in models which are “heavier” (more control points, etc) than is needed to represent the geometry.
Best practice is to pick an appropriate tolerance to begin with and stick with it. If in doubt use a smaller tolerance as the results will be useable, just possibly a bit “heavy”. Too large a tolerance can result in needed to redo work. Changing a tolerance while modeling only affects work done after the change. Previous work is not affected. If the larger tolerance caused problems then the work with problems will need to be redone.