I did a sweep2 along the upper and lower black lines using the shapes. Why would I get twists in the resulting surface?
HI Jim- with closed section curves you need to make sure the seam points and direction arrows line up correctly.
-Pascal
Why would they not be lined up in the first place? They are all lined up parallel.
You get to click on the arrows to drag them to a more logical location to line things up nicely or flip them (command line option) - generally it works well to try to line them up to corresponding end points on the section curves.
If you don’t line up the direction arrows, things go wonky.
-Pascal
But why are they pointing in 4 different directions in the first place? Especially because the direction should be constrained by the rails.
The directions of the cross section curves are entirely independent of each other and of the rails. The seams and directions are determined by how the curve was initially drawn. The first and last point of a closed polyline is where the seam will be, and the direction will be determined by where the second point is.
You can always check the direction of a curve with the Dir command and adjust the seam with the CrvSeam command. The sweep commands will also pause for you to adjust your seams and directions to match properly.
Thanks, I’m adding that to my Rhino notes file.