You used to be able to build an artificial double slope and then you could ctrl click to place a tunnel entrance. this no longer seems to work and it seems there are a new set of conditions that must be met in order to build tunnels. can someone more familiar with this please enlighten me.
It's a known bug that hasn't been fixed yet.
It is a bug from Standard (https://forum.simutrans.com/index.php/topic,20718.0.html). Thanks to Vladki and prissi, it was fixed there last week, but the fix has not yet been brought across to Extended.