Good point about "boring vs blasting". I didn't think about that. I remember reading about the longest tunnel in Japan between Honshu and Hokkaido (Seikan Tunnel). I recall that it was entirely hand drilled due to unusual soil conditions. I wonder if that would still be true today with state of the art tunnel boring machines.

   > Being two separate tunnels, it also needs twice as much excavation work.

Yet another great point. At some of the Toei Oedo stations, you can see a miniature model of the weird overlapping twin tunnel boring machines. So, in theory it is two tunnels, but in practice, it was dug as a single, weird overlapping twin tunnel.