Increasing the diameter proportionally increases the exit pathway through which you can shift earth out of the tunnel. Obviously there are complexities in the f

Increasing the diameter proportionally increases the exit pathway through which you can shift earth out of the tunnel. Obviously there are complexities in the field that I'm not aware of but these seem like problems you can engineer around. It's really surprising to me that in 2019 the limiting factor in a billion/trillion dollars industry like transit is something that could be solved with a better conveyer belt. To ask OP's question another way: many areas of technology and engineering have improved rapidly over the last decade or three. Have TBMs been keeping up?