Yeah this is all very odd and everyone is lapping it up. He claims to want to reduce tunneling costs by a factor of 10, but starts by quoting a very high benchmark of $1 billion per mile which is quite easy to beat. Then he reduces diameters to something much smaller than all the expensive mega projects, into the range of something where technology is well established and costs are already 1/10th of his quoted $1 billion per mile. But then for some reason the solution has to involve putting cars into these tunnels. If what he thinks he is on to is viable with cars in those tunnels, it's even more viable with trains carrying 1,000 passengers each coming every 3 minutes. So why aren't we building these subways with trains?

The largest part of the problem to solve here for the US is not the technology of tunneling. It's the process of engineering and building infrastructure like this that seems to become way more expensive in the US compared to places like Spain. I do believe he could reduce costs by vertically integrating the whole thing. But then we should ask why don't we do that anyway?