The highlighted region is formed from two arcs and two lines. As you can see from the coords, everything lies in the xz plane.
The outside contour was created using the offset tool on the appropriate edges. (Bonus question - why does the offset tool fail on the highlighted face? I’m trying to offset .75", but it allows only about half that.)
Now look at this zoomed in view:
The selected line, in blue, intersects the upper curve in the figure, but not the lower; selecting the line at the lower endpoint selects the entire line - it’s not two separate segments. In other words, the intersection with the boundary is not being recognized.
Presumably the problem is that something is not truly planar, but I don’t know how to draw this and get it all to lie in the plane.
TopRail.skp (26.7 KB)