In sliced underground view there are two problems now:
(1) walls facing back are not displayed
(2) walls are drawn on top of tunnel mouth if the tunnel is at the slice level.
Patch attached. I do not knwo whether it breaks something else. Please test.
Confirmed bugs however patch doesn't work correctly with double heights. Will investigate more tomorrow if I get a chance.
Committed a better patch, see r 6807. It should also work properly with double height pak (tested with pak128.Britain).
Great thanks :)