It's easy to produce this:
1. Build a two level heigh artifical plateu
(https://lh6.googleusercontent.com/-5h7sARtOvQQ/T3hUI6biqmI/AAAAAAAAKo0/PpCGv3GoQkQ/s912/simscr77.jpg)
2. Build a tunnel on the secon level in it with ctrl
3. Drag the tunnel off the plateu
(https://lh6.googleusercontent.com/-HKTfAsw2jK8/T3hTchS7_-I/AAAAAAAAKos/j2E1o7e4FD4/s912/simscr76.jpg)
Other examples:
(https://lh3.googleusercontent.com/-jIJH5gnzgZc/T3hRa9U_rvI/AAAAAAAAKoY/tWZr3AbedOQ/s912/simscr74.jpg)
(https://lh5.googleusercontent.com/-KAHelu0ROOM/T3hTXwmxCjI/AAAAAAAAKok/CHnUZ9CtpwY/s912/simscr75.jpg)
It occurs in the latest nightly (r5599) and the experimental version 10.11 too (i tested it in these)
Confirmed in sliced height mode. Working on fix.
simconst.h
// define this to disallow the harbour tunnel feature
//#define ONLY_TUNNELS_BELOW_GROUND
Somehow this got commented out at some point, years ago I think? If ONLY_TUNNELS_BELOW_GROUND is enabled behaviour is as it should be.
I think that with sliced mode, building underground should be rethought, even for the ease of GUI.
Normal tracks should be the same under and above ground with overprice the same for all of them when building underground. Entrance underground should be automatic when in sliced mode.
And entrances on vertical slopes should be allowed, one of the heavy reasons for it being that an overground diagonal track cannot keep diagonal if going underground...
Actually bug (which is same as old habour tunnels) happens in normal underground mode too. Was this left in as a feature - I thought it was removed?
It was removed, at least this was the intention. It was just shortly after underground mode, I think?
That was what I thought, however the code to allow it is still in trunk (and as can be seen, enabled by default).
Iirc there was consent to leave this "feature" in the code. But I think this was before the underground slopes came. So there is no need to keep it imho.
removed this with r5608.