Tunnel height restrictions

[Wiimeiser 436]

What exactly are the conditions required to drag a tunnel? What do we know about their restrictions? According to these:

There does seem to be a depth limit for tunnels, as I couldn't drag one under the ridge there...

EDIT:

Also seems to vary depending on the network:

As can be seen here it works with road (or maybe something glitched, given that the snow's gone...)

[matias93 7,535]

All of this is defined on the terrain controller: the slopes of the different networks, the minimum height of the tunnels, the level of admisible inclination of such tunnels, etc. I know that some guide exists, but now I cannot remember where is it. Instead, I would make a backup of the terrain controller (or the slope mod, whatever loads after) and would tinker with the exemplars on the reader until getting the desired effects.

[HayleyRenaNemeth 465]

using the single rail tile method and leveling a 2w x 5l landing will allow you to build a tunnel. Its less about the height difference at each end but the slope of the entry points at each end its gotta be steep enough that the game will op for a tunnel. You'll see the path turn from Yellow to Green. Unsuitable Grade for Construction typically means that the path is steeper than what your slope mod will allow. As if you don't have a slope mod you will rarely see this.