[yongjik]

Not so sure about that. Some subway tunnels are barely wide enough to fit a single train. I don't think a car tunnel can be much smaller, even if you only allow sedans and somehow find another way to send an ambulance in case of emergency.

[wahern]

I think all the subway tunnels in the U.S., including those in Boston, Chicago, and NYC, are >24 feet in width. The London Underground's deep-level tubes are less than half that at <12 feet in diameter.

That said, I don't think American cities could run subway cars as small as in the London Underground even if they wanted to because of the Americans with Disabilities Act (ADA).

Also, it might be difficult to make cars that small that pass muster under Federal safety regulations. AFAIU, Federal regulations basically require subway cars to be built like tanks.

EDIT: >24 feet just seems too wide for the old, deep tunnels in the U.S., but I can't find any references that aren't from books at the turn of the 20th century, which have very inconsistent numbers. I said >24 because that seem to be the width of old cut-and-cover tunnels and newer deep tunnels. I'm going to go out on a limb--based on random sources--and guess that the really old tunnels still in use might be closer to 16 feet in diameter. Feel free to correct me.

[capkutay]

The most expensive part of subway construction - both time and cost - is building the stations...which get significantly more expensive the lower they are underground.

Also modern subway tunnels are typically constructed with a larger diameter than what the Boring company supports.