Hi jake, so for original question on why sweep is not working well there - the way one rail sweep works is it moves the profile along the rail rotating it by the rail curve's tangent direction. You can get a kind of preview of what that is doing if you use Transform > Array > Curve on your profile, which will generate this:





So notice there how the sections are bunching up and crossing over each other there? That's what is making a poor quality surface result.

For one-rail sweep it's kind of a tube-like construction mechanism where you can't have a profile curve go off by a larger distance than the radius of the bend, otherwise sections will bunch up as shown above.

Instead of the profile curve rotating with the curve tangent, you would instead need it to pivot around its own endpoints. I want to make that kind of pivot an option for sweep in the future but there is a fairly similar command now called rail revolve, under Construct > Revolve. It allows for one profile curve, one rail curve and then also a revolve axis. The profile will pivot around the revolve axis while scaling out to meet the rail. So since it has a pivoting action it could work for this kind of case.

Anyway hope that helps explain what you were running into originally.

- Michael