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...

You're missing the biggest one - evacuation. The reason many tunnels are oversized is to allow walkways for people to evacuate using in an emergency. This drives the size up a fair bit.

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

Seems like you missed this part.

You might not be able to fit a full-sized train in there, but don't forget the original New York City Subway had to fit in some relatively tiny tunnels: http://www.nycsubway.org/wiki/The_Construction_of_the_Harlem...

Subway trains are often engineered to specification, so if you have a tunnel of a particular size the manufacturer can make a train for it.

The precursor to the modern subway was a strange pneumatic system that would fit in that tunnel as well: http://www.nycsubway.org/wiki/Beach_Pneumatic_Transit

The London Underground deep-tube lines run in 11.5 to 12 ft diameter tunnels. The Glasgow subway uses 11 ft diameter tunnels.