This is a solved problem. The obstacles to better public transit are political and economic, not technological. Arguably, you're also talking about SpaceX. Lan

This is a solved problem. The obstacles to better public transit are political and economic, not technological. Arguably, you're also talking about SpaceX. Landing a rocket on its tail is something we'd already done. The rocket engines SpaceX uses are tried and true technology. The Falcon rockets are a technological tour de force, but really that's all about implementation. The big barriers were really political and economic. A situation had arisen where an industry had become stagnant, and everyone was scared off from competing. It wouldn't surprise me if the same applied to tunneling.