Are you allowed to 'cheat' a bit.
When rail sweeping along helix (or anything like that) the ends are not horizontal/vertical, meaning the end of the rail sweep will bash into the start of it, causing self intersection.
A couple of ways (there will be more ways), click the helix with the edit tool, in the inspection bar change the turns to 0.999 (or 0.99), or use select tool and alter the turns in selection properties palette or properties dialog.
Rail sweep with 'rigid' not checked, this will leave a small get between start and finish, use the quick pull tool, or facet edit tool to move the end of the sweep filling in the gap.
Alternatively use the revolve tool with number of coils set to 0.9999, and which ever spiral pitch is required, obviously setting clockwise or anticlockwise as required