> Though if they really dug that much tunnel under a city in such a short time, maybe there's an actual innovation in a better way to make subway tunnels? Or m

> Though if they really dug that much tunnel under a city in such a short time, maybe there's an actual innovation in a better way to make subway tunnels? Or maybe tunneling speed simply isn't the bottleneck so no one ever tried going faster. If it takes 3 months to dig out the tunnel but 6 months to fit it out then you've gained nothing.