They did. Modern safety standards make them uneconomic (you're required to provide a safe evacuation route for disabled people). It's a regulatory arbitrage just like hyperloop (if you make a train line to car safety standards you will have much higher capacity than a train line that follows train safety standards, who knew?)

This won't have anything like the capacity of a train line.

Even if they do get their quoted 30 second headways and manage to cram 16 people into a car, and they run 20 hours a day like they're claiming, that's only 38,400 trips per day. Realistically it would probably more like a third of that. The Red Line alone averages 250,000 trips per day.

Uh, how is that going to get permission to run, though? Regulators are past the "wait for a tragedy then say we told you so" thing.

Firstly most of the London Underground was built cut-and-cover which is no longer practical in modern cities. Secondly, this will be much smaller than even the Tube. Crossrail’s tunnels will be about 6 meters, most existing are more like 4. BoringCo is talking about something like 2 meters. Note that a 2m tunnel requires excavating 4x less earth than a 4m tunnel and 9x less than a 6m tunnel.

Most modern tunnels built for existing transit systems are built to fit existing rolling stock, so the idea of smaller tunnels gets ruled out early on. In the past, smaller tunnels were used. In fact, one of the most extensive small tunnel transit networks was in Chicago: https://en.m.wikipedia.org/wiki/Chicago_Tunnel_Company

> Firstly most of the London Underground was built cut-and-cover

The small-diameter tunnels we're talking about were built with a tunnel shield, which is the hand-operated predecessor to the TBM.

https://en.wikipedia.org/wiki/History_of_the_London_Undergro...

Most of the London Underground is above ground.

Of that underground, I Elise the majority is the deep tube bored lines (pic, jubilee, bakerloo, central, northern) rather than pretty much just the circle that's cut and cover.

> Most of the London Underground is above ground.

Technically true, it's 45% that's underground, which includes 32km of cut-and-cover lines and 150km of tunnels out of the 402km total.