[ArchitectAnon]

Tunnel boring machines aren't that expensive per km. They are robotic worms that take a few dozen people to operate. If you do cut and cover you will end up with a similar problem to the tram system in Edinburgh where there were a lot of pipes and cables under the street that were uncharted and not as deep as everyone expected. Dealing with old pipes and cables is very delicate; it involves a lot of careful hand digging with shovels and picks and then careful propping to avoid damaging the old clay drain pipe / ancient electricity cable / iron gas main / water main etc. All of these can be lethal if they are damaged and they all have to be worked on by hand. Super expensive.

[xxpor]

The half and half solution (which IIRC the report suggested) is cut and cover stations with bores between stations. Digging out stations from the inside out is what's really insanely expensive.