Definitely tunnelling through rock is a more straightforward proposition. Bertha famously hit an allegedly uncharted steel pipe part way through and had to have

Definitely tunnelling through rock is a more straightforward proposition. Bertha famously hit an allegedly uncharted steel pipe part way through and had to have extensive repairs. Clearly tunneling under a city is a very different proposition from tunneling under a mountain range. And I'm willing to bet that a significant part of the cost is not per km, nonetheless it's just another example of the crazy cost of civil engineering in the US