> The bigger factor behind these crazy-deep tunnels seems to be pre-existing infrastructure. If you're doing cut-and-cover, you're digging out all of the infras

> The bigger factor behind these crazy-deep tunnels seems to be pre-existing infrastructure. If you're doing cut-and-cover, you're digging out all of the infrastructure first and then replacing it -- it's highly disruptive. While if you're boring a tunnel, you need to go below everything that already exists -- sewers, water lines, deep building foundations, pre-existing subway lines that it crosses underneath, etc. Makes perfect sense! And yeah, it's yet another case of path dependence: we'd build far different cities if we had the option of designing "from scratch" rather than layering retrofit over retrofit under retrofit. In an ideal world, we'd have tunnels connecting B1/B2 of every major building in a city, and many people's trips would be elevator-transit-elevator.