Hi Ryan, yeah your current part cannot be capped because it is just a open surface fragment, it is not closed in any direction.

In order for a shape to be automatically capped it has to have an edge that makes a closed planar loop which then can get sealed at the end.

So for example something shaped like this:



Can have caps applied to its ends to seal it into a solid.

Your current model is more like the following:



That only has a surface going 90 degrees instead of a full loop - it's not formed enough yet to be capped, you have to build more pieces and join them together before you can make a solid out of it.

MoI doesn't really automatically know how to turn such an unfinished fragment into a solid, it's not clear enough for it to do it automatically, you have to build it out more and fill in more surfaces to define the structure some more.


> If I take the rail line curve that I've drawn and draw two straight
> lines (almost at a right angle to each other, connecting the two
> ends of the current rail) that would close the rail in sort of a
> rounded triangle shape,

Well, maybe yes if your end result is intended to be something shaped like a partial rounded triangle shape... But you have the other half mirrored over to the other side, what is your intention for your final shape, is it supposed to be 2 separate rounded trianlge things or is this supposed to be one full shape that has a bridging connection between the halves?

Maybe you will need to give a rough pencil sketch of what you are trying to accomplish overall, it's not at all clear to me what kind of overall final shape you are trying to build.

- Michael