Hi Dave, yes 2 rail sweep is a kind of general freeform surface construction mechanism. It doesn't make 100% precise geometry it makes freeform surfaces as its output.

If you use Edit > Separate on your shape and then turn on surface control points for the surface you can see that it's kind of stretching out between the 2 rails, so yes it's not symmetrical and uniform like something made up from a cylinder and torus would be:



As the profile moves between your 2 rails it gets stretched out in one direction and so a rotated copy of it will not exactly align to other pieces of itself as an exact pipe that has a circular cross section would.

- Michael