Hi JTB - that is a limitation of sweep right now. It isn't able to miter the corners of a multi-segment rail curve like that. It is currently handled just as if each line was separate.

Eventually I want to make it work like you expected, but it is quite a bit of work to make it work reliably - I started to work on it when I was doing sweep but it was difficult enough that I decided to postpone it, I don't think I will be able to do it until after V1.

In the meantime, using 2 rails like Jesse shows is probably the best workaround. It's also possible to do it with your existing curves by using Rail Revolve.

- Michael