Hi Val, Actually I don't think that the simple sweep option in Rhino is intended to be used for this kind of thing.

At the first glance the shape in Rhino looks ok, but if you look closer you can kind of see that the sweep in Rhino is messed up, there is bulging and a kind of slanting happening in the sweep, causing some non-constant thickness. Even though it makes corners there, it is not an accurate sweep (below is the sweep with simple option in rhino):



When you compare to the 2-rail sweep in MoI, I think MoI gives much more proper results. The shape of the sweep is correct, it hasn't slanted or compressed it in an extra way at all, but there are no corners. Below is the sweep in MoI for comparison to the above:



One of the problems with this sweep is that the rail is not actually perpendicular to the plane of the start shape, this kind of gives it a slight slant throughout:



With rails that are exact copies of one another, I'd actually recommend using one-rail sweep for that instead, with the selected curve below as the profile, and the long one as the rail. Switch the "Twist" option to say Twist:Flat - that will cause the shape to only rotate around the zaxis and not rotate in other ways around the rail curve.



Then the limitation is that MoI won't miter the corners for you automatically, you need to miter them manually right now. This is done by extruding the ends so they punch through each other:



Then select the 2 extensions, run Edit / Trim, push Done since you want to cut them with each other, and then pick the parts to throw away. That will leave you with the mitered corners which can be joined together:



For those results in MoI I also edited the rail curve so that its tangent direction was exactly perpendicular to the profile's plane.

- Michael