Here's an example using rail revolve - You actually just need the bottom profile and the rail for this. Start by selecting the bottom rail, then run Construct / Revolve / Rail revolve. (note that this is a sub-option alongside the regular Revolve command).

Then pick the rail curve, then pick 2 points to define the pivoting axis of the revolve, which you want to snap on to the ends of that polygon:




EDIT: oops - I thought I made a mistake here, but I think that this actually does give you the right shape that you were looking for.

You'll need to build some planar surfaces to finish this shape into a solid. To do that, construct the first one by selecting that bottom profile and running Construct / Planar. That will build a planar surface there. Select that planar surface and the result of the rail revolve and then use Edit/Join to glue them together. Then select that "almost solid" piece and run Construct / Planar again on it to fill in the last remaining hole and make it into a solid.

Let me know if you need any help with these final steps, that is using Construct/Planar in 2 different ways - if you have a planar curve selected it will build a surface for that curve, or if you have a surface or joined surface selected with a planar opening in it, it will seal that off. But it has to be a simple planar opening, that's why you have to seal off one side of this shape manually before that will work.

- Michael

And by the way, the reason why this other one next to it worked fine:




Is because the path bends more gradually there and the size of the profile is smaller than the curvature of the path, so it avoids that "folding over itself" problem that I showed earlier.

Hope that helps to make some sense of what you were running into at least!

- Michael