I don't like this idea. If someone puts a tunnel on top, now readers wouldn't be able to know which tunnel the terminal was associated with -- that thin dividing line's presence/absence is really easy to miss. There isn't a lot of space in that to do more graphics to enforce the association.
To me, it is too minor a diagram cleanup gain to be worth the readability problems, not to mention the increase in stuff in the tunnel's right click menu.
I would say no, but the idea stays open to see what feedback it accumulates.
