Signals in tunnels and bridges are already done as a patch. Look in JGR's patchpack.
Underground station, cave etc idea is good, and is something which would need implementing. However, it would need an underground view (like SimCity 4) and so a rewrite of the map array. Thus, it is possible, you will be able to load your old savegames with the new map array, however, there is total irreversibility. Also, rewriting the map array is hard, although i'm sure there are some people (such HackaLittleBit, that done the signals in tunnels patch and cirdan, who did NMF) which may be able to do it.
well visibility is another issue
butt first first
if we could get the cave function would be nice even if we build in blind
that why i said the toll like copy past, or even better cut paste will be handy,
build it somewhere you can see and then move it into place.
as i see ti now the landscaping tool effect the whole column of area
to make a cave we will have to say to ignore the above area and only apply changes to the tiles under it
which if possible then we can have the opposite tool and dig in things
dig in an area build what you want to do and then dig it in. in this case the landscape tool will not affect the area bellow only the area above
as for visibility
an easy way around is to draw part of the map
meaning if the map lets say has a height of 16 tiles and you want to see at tile 12 then ignore the 4 tiles or in simple terms treat the game as if it had only 12 tiles and not 16
or maybe with invisibility transparency?? make the above tiles to be invisible??
and this function goes beyond the signals in the tunnel since now you can custom make tunnels of any width and crossing and even put depot inside them
for beginning i would not mind building blind if we can have the feature
visibility can come later on
if we can get visibility then it will be awesome since we can really build underground
imagine a late game with a big city with a complex multi level metro running in it