Makes sense, thanks.

In the unlikely event someone manages to find this topic via search, the solution to avoid rail planner placing foundation on Vulcanus is to simply to hold the control key, which will route around the lava like it does before foundation is researched.

When you disable the "build terrain" permission for a group, that group can no longer place landfill or foundation directly (working correctly).

However, you can still place landfill by shift+click'ing a building over water/lava, which automatically places the ghost landfill/foundation tiles as ...