The funny thing is, the actual industry is going in the opposite direction, because it turns out that a single big tunnel is in practice cheaper than a bunch of small ones. Conventional deep-bore metro lines consisted of two tunnels about 3.5 meters in diameter, but if you look at _current_ metro projects, the Dublin metro is using a single 10 meter tunnel, the newer Barcelona lines are using a singe 12 meter tunnel...
The expensive bit of building metros isn't actually boring the tunnel, generally, not anymore. It's everything around that; securing a route, the disruption while it's going on, etc etc. So the last thing you want is to have to do it twice.
The expensive bit of building metros isn't actually boring the tunnel, generally, not anymore. It's everything around that; securing a route, the disruption while it's going on, etc etc. So the last thing you want is to have to do it twice.