[Bug] Can't build underground signalboxes

[Mariculous]

What happened?
Currently, building underground signalboxes seems to be impossible.

How to reproduce?
1. start a map without timeline or at some time where underground signalboxes are available e.g. 1978
2. switch to sliced view mode.
3. try to build an underground signalbox at a grund that is at least two levels (a full height) under ground.
4. You won't get any warning but it simply won't work.

A minimal reproduction case map is not pretty helpful in this case imho.

[BAC1-11]

I sometimes find this too. Though other times for seemingly no reason it does work. Very bizzare issue. ATM I just avoid underground signal boxes and construct them above ground, but it is an issue.

[jamespetts]

It seems that I missed looking into this at the time that it was posted. Somebody has posted on Discord that this occurs whenever a building is above the signalbox. I am posting this to remind me to look into this when I get an opportunity.