Wow you 've got a plan

. That's cool.
0) Before opening blender, you should look for documentation. You need tons (100 is a minimum) of photo references, maps, etc about sewers, metro, underground facilities, etc. You can watch on youtube video about underground to see how it looks. be CURIOUS !!!
1) You can start to think about what can be cool to include. You can also see other games how they made similar environment.
You need to choose a color palette. A color palette is what is the main color that you will use to make this track. Of course a red barrel will be alway red. But the general atmosphere needs to have one or two color.
2) You have to make concept art. Where buildings, pipes, etc will be located ? You can also make a very basic track with only the path and blocs to show where buildings might be. You can see if the gameplay is interesting, what do you really need to do (that's the state of harvest. Currently there is only the building very quickly done and the track).
Also if you want to add something special (like water flowing) you need to look for documentation about this effect. How it's made by professional games ? You can try to make it in a mockup. That's exactly want I did with water reflexion on the mine. I won't necessarily use water reflexion in this part or even on this track. But now I know exactly how to do

.
(example of water reflexion in the mine)

3) You can start to make the track

. Remember, failure is ALWAYS an option. It's difficult to make a good track. But we learn from mistakes and that's how it works. Try to improve yourself, for the moment I suggest to focus on modelisation and texturing. Lighting will come after
