I am primarily interested in viewing and designing the underground facilities and infrastructure for large Urban Development projects.
If not yet possible, are there any plans to make these things possible in CE 2017 for example?
Currently in CGA one can directly query the world elevation using the scope Shape Attribute .
With this information the scope can be translated to a certain height.
This allows for exact modeling of subsurface structures like wastewater networks.
As shown in this thread https://community.esri.com/message/666944-re-detecting-terrain-intersetion-with-a-shape-to-achieve-m... the relation of the scope to the ground can be obtained by adding shapes onto the terrain. I hope in a future release there will be a simpler way to do this. For visualization the terrain can be made transparent.
CityEngine 2017 will offer new capabilities for CGA to query for collision with other models.
Since there is a wide demand for subsurface structures, there are plans to offer more CGA rule sets for such applications. Please check GeoNET for updates.