>>> 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 $$$ :/
I was thinking same direction ---->---->. Though as the other poster pointed out, often you are going under something and so you can't do that - the English channel did what you propose and there are thus tunneling boring machines under the channel.
What I'm picturing from your statement is two TBM drilling towards each other like ---> <----.
Which would require the digging of a third hole where the TBM "drill heads" meet. After every TBM dig, the drill heads are left at the end of the tunnel b/c they are bigger than the new tunnel (as the head digs the tunnel, workers install support structure to prevent a collapse, so the just drilled tunnel is smaller than the being drilled tunnel). A 3rd dig would be required where the two TBM drill heads meet to retrieve them and connect the tunnels. That's a lot of $$$ :/