Has been an issue since the Railing tool was introduced. You cannot do a closed path. The closest you can get it to terminate on an edge, removed the unneeded post, and set the rails and toprail to have zero end offset.
I'm not sure I understood the process exactly. I tried to do it but it was looking like a big kludge. Changing the offset value to zero didn't seem to have any effect. I got something close just by dragging elements in the edit window.
Perhaps a video will make this process clearer. If it's not asking too much. Thanks in advance