Thanks for the tip, Michael.

After using Archipelis, I decided to see if I could build in a similar way in MoI. I came up with the similar approach to yours. But yours seems much better, because I was creating the two rails seperately and then sweeping an ellipse through them. Which meant I then had to go deal with the capped ends.

Anyway, after playing around for an hour, I came up with something fishy. I think I've learnt finally learnt how to build in MoI. At last! :)