>>> 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 $$$ :/