Which means the throughput per tunnel is more expensive, right? A tunnel with a half diameter will have a quarter the throughput (i.e. the ratio of circle diameter to area). Which is why TBM tech develops towards bigger machines, not smaller ones.

Why does a hyperloop fit in a smaller tunnel than a train? Doesn’t it need to do everything a train does, plus it’s wrapped in a vacuum tube?

As far as I understand, the original proposal relies on a) no safety egress walkways in the tunnel and b) a more cramped vehicle profile.

I don't know how as-actually-built hyperloop compares to rail tunnels.

But that's offset by it managing much lower peak passenger throughput, so you need more tunnels...

You don't need more tunnels -- you just limit your passengers to your most profitable, and make a luxury transit system rather than a mass transit system.

This is what Tesla did by starting with the Tesla Roadster then Model S then Model 3... and it's what the Hyperloop aims to do. Launch as a small throughput luxury product and if it's profitable then it doesn't matter if it ever expands beyond that or not.