Hi photo - so the sweep in that case is behaving odd because it's getting "auto place" mode activated.

Auto place mode is a method for using sweep where you can have the profile curve drawn just flat on a world plane somewhere away from the path curve and then Sweep will move and rotate the profile on to the rail curve for you.

The way that auto place mode is turned on is that sweep looks at the profile and if it totally outside the bounding box of the rail then auto place mode gets enabled.

I've been meaning to add some control in to make it possible to turn auto place mode off because sometimes like in your case here it gets enabled when you are not expecting it.

For now if you move the arc curve just a little bit to the right you should find that it then works like you were expecting.

Some other things you could do would be to make a slightly longer arc so that the whole arc was not totally to one side of the rail's bounding box, or use offset on the rail to make a larger rail running more down the middle of the arc, or you can also use Rail revolve for something that pivots around a central axis line like this instead.

There's some more description of auto place mode in the six legged pod video here:
http://moi3d.com/2.0/docs/pod.htm


Let me know if you are still stuck.

- Michael