Hi Michael,
Attached is example of what I used. Projected line on to spheroid to use as rail. I initially tried the scaling rail approach but found it very fiddly, as if you duplicate and rotate the main rail there will be small sections at either end that aren't covered by scaling rail so you get a step. If you try and adjust for that I ended up distorting the curve shape.

I have had a quick go with your suggestion of a 2 rail sweep, with maintain height off and a bit of tweaking I think that may do the trick!

Thanks
Andy