Hi fleetfoot - you're using "auto place" mode there for the sweep and it's doing something like placing the profile at an angle to make it perpendicular to one of the rails or something like that.

Try placing the profile directly touching the rails like this:




That will avoid "auto place" mode from moving your profile into place itself, when the profile touches the rails like that it will be used directly from where it stands and that should give you the result that you were looking for.

- Michael