Comment by Animats

Now that's a useful comment.

Here's Crossrail's video explaning how their TBMs work. This gives enough detail that you can get an understanding of what's going on.[1]

There's so much variation in geology. The Seikan tunnel to Hokkaido had huge variations in soil and the project went very slowly. Eurotunnel is mostly through chalk, and went well. The Gotthard Base Tunnel in Switzerland was a long grind through hard rock well above water level. That's a slow process, but was reasonably uniform. SF's BART tunnel, unusually, was built onshore in sections and lowered into a dredged trench. NYC's Water Tunnel #3 was in hard rock, and that project went very slowly.

Many things can go wrong. Hard rock may not be competent rock capable of supporting itself. The Tom Lantos tunnels south of San Francisco were through a small mountain of loose shale. One tunneling project in Japan hit an underground river.

The most advanced underground rail project today is probably the Tokyo-Osaka maglev line.[2] 43 kilometers of this is already operating, and most of the route is in tunnel. Tokyo to Nagoya is scheduled to open in 2027. The connection through to Osaka was scheduled for 2045, but the Government has decided to accelerate the program and get it done sooner. Here's what the ride looks like.[3] 500km/h. Working now.

[1] https://en.wikipedia.org/wiki/Ch%C5%AB%C5%8D_Shinkansen

[2] https://www.youtube.com/watch?v=z38JIqGDZVU

[3] https://www.youtube.com/watch?v=ltqp4McM2wY