Hi JTB - do you mean you want to fill in the top part of that shape?

You can use Construct/Sweep to do that. Start by selecting the curve out in front, like this:



Then run Construct/Sweep. In the upper-right corner, it will prompt you to "select one or two rails". Now select the other two curves and push Done. That will generate this shape:



Please let me know if that was not what you wanted to do.

- Michael