So with all geometry in Top View, an arc or a circle, with center point selected, will Sweep a rail curve, (of large enough radii), SOMETIMES produces an envelope.
More testing needed...

Sometimes an "envelope" is produced, sometimes a 3D sweep occurs.
Placing the start of the rail curve at the center of the circle yields 2d.
Then moving the rail curve causes change to 3D..


Maybe I can use this method for a project, or not.

- Brian