There are a few reasons that I'm aware of that new projects typically bore larger tunnels versus a 12ft bore that would reduce construction and operational costs:

- Logistically, it is difficult to get a cement truck into a small tunnel, as each truck would need to back out of the tunnel. You end up offloading into smaller cement trucks that can turn around, which is the headache that the Eglinton LRT tunnels is dealing with now

- Need space for a level floor, conduits/tunnel infrastructure and evacuation walkways

- Air flow. The rail car is an effective cork. Small tunnels require more ventilation and draw more power to push that air being compressed around -- unless the underlying truck is going to be propelling itself via the air

Barcelona's system is apparently the new gold standard for how it should be done [1]

They were able to build it out at ~$39 million/km[2]

[1] http://www.skyscrapercity.com/showthread.php?t=1866842 [2] https://pedestrianobservations.wordpress.com/2013/06/03/comp...