[jcranmer]

> On the other side if they don't have to move millions of people to the same hub but rather move a much smaller number of people, they can afford to dig smaller tunnels and stations which are way more easy and fast to build.

The actual cost of tunneling itself is generally fairly cheap--somewhere around $50 million / mile. The expensive part is the stations. You can probably save money without having to build mezzanines, but the lower utilization of the tunnel and the greater number of vertical access shafts needed (not to mention the challenges inherent in moving through that very crowded portion of real estate) is probably going to cause cost blowouts compared to subways. Particularly if you design the tunnels to move cars, not people (SOV cars being about the worst use of space possible).