Hi Andy, can you please post the .3dm model file that corresponds to your screenshot?

The Loft command expects to have a set of cross-sections, not any rails. You're getting a chaotic result because it's trying to use the rails as cross sections.

If you want to do a loft with guide rails use the Sweep or Network command for that not Loft.

But if you can post your model file it would make it easier to give you a recommendation.

- Michael