> A few meters a day sounds slow, but if you are going long distances you can scale by buying more TBNs putting them in a line and having each meet up to the next.

This would be impractical for most real-world projects. First, digging the access pits to get the TBM in and out are a significant fraction of the project cost - access pits are really expensive, and require a big space, so you don't want to dig more of them than you have to. Second, you're normally digging a tunnel (rather than building on the surface) for a reason: you're going under something. That something might be a body of water, it might be a mountain, or it might be a dense city full of valuable real-estate that you didn't want to disrupt with a cut-and-cover operation. Either way, it's going to be really hard to dig an access pit right in the middle of it.

20 meters a day is 4 km/year assuming you only work weekdays. (This is a bit faster than average, but not unreasonable). In your city you only need a pit every 4km, which is a lot easier to find, you don't have to be exact (in fact you don't want to be because you probably need more crew to pull them out at the end of the tunnel), so you can find an outdated building to buy for cheap someplace along the route. Or you can do a two year dig with 8km access holes. Don't forget that you need a station someplace in the city - your large hole to get the machine out in the middle of the city should be planned where you need the largest station anyway.

Of course for mountains and large water bodies you are correct. The English channel left some TBMs in the middle so they could get done faster - at much higher expense.

>>> you can scale by buying more TBNs putting them in a line and having each meet up to the next

What I'm picturing from your statement is two TBM drilling towards each other like ---> <----.

Which would require the digging of a third hole where the TBM "drill heads" meet. After every TBM dig, the drill heads are left at the end of the tunnel b/c they are bigger than the new tunnel (as the head digs the tunnel, workers install support structure to prevent a collapse, so the just drilled tunnel is smaller than the being drilled tunnel). A 3rd dig would be required where the two TBM drill heads meet to retrieve them and connect the tunnels. That's a lot of $$$ :/

> Which would require the digging of a third hole where the TBM "drill heads" meet.

In most projects it's called a station.

http://www.crossrail.co.uk/construction/tunnelling/meet-our-...

Occasionally you'll have dive sites that are not at/near stations, but not that often.

https://www.sydneymetro.info/station/blues-point-temporary-r...

https://www.sydneymetro.info/tunnelling

I was thinking same direction ---->---->. Though as the other poster pointed out, often you are going under something and so you can't do that - the English channel did what you propose and there are thus tunneling boring machines under the channel.