On: Tunnel Boring Machines (2018)

[mrep]

Says who? I grew up in chicago where not only do we have an elevated subway [0] but we also have some double decker streets such as wacker drive [1].

[0]: https://en.wikipedia.org/wiki/Chicago_%22L%22

[1]: https://en.wikipedia.org/wiki/Wacker_Drive

[bluGill → mrep]

We can of course build 3d. However the costs of building 3d is vastly more than building 2d. Thus we generally prefer to build mostly on the surface unless forced to do something else.