sadly i'am just able to make you look sexy with PS, not a 0111001 person.. anyone interested to make an mod?
i could make decent images / sprites, in what ever format is required, if someone is willing to script it.

as far as i know it can't be that hard..
it's like adding 8 (+the belt / pipe stuff) ofc.) new tree's / rocks to the game, right?
shouldn't be destructible etc. but i guess that would be just some minor changes..
the mining drill train is far away from being a beauty, i just thought now that we can use equipment on vehicles it would might be a possibility's to make tunnels without blowing up the planet!
it would take some brainstorming, what could be and what not, maybe we could first make it work on the map editor & build a scenario with it. so the community can see for it self how it "feels".

the tunnel could be possible in the same way "Factorisimo" works, but i'm not quite sure if the pathfinding AI would bug out by that. could trains pass trough ? what about our beloved biters? could they enter? is that even a thing to consider?
i'm against a underground base, should be limited to non connecting tunnels, in fact there shouldn't be much more then train tracks inside maybe a lamp or two.^^

so long & thanks for all the fish, n00dles