Tested with both pak128.Britain and pak64.
When using sliced underground view, building a rail tunnel, I could only go to a certain level, but using full underground view, I could continue several layers deeper. At a certain point, using full underground view, I could lower the tunnel one section further but I couldn't build on to it.
To test: Start a tunnel and then using sliced view, keep extending the tunnel deeper and deeper until you can't go any further. Then, using full underground view, carry the tunnel down deeper until you can't build on to it any more.
The arbitary limti was set to sea_level-5, since no simutrans ocean is deeper than 4. Allowing to the lowest tile in the map will require some additional logic.
EDIT: in in r7424