Comment by newyankee

I understand how reducing tunnel diameter by half reduces cost by a factor of 4. I can even imagine that using electric motors, batteries and few other optimizations might give an energy efficiency of 2-3x compared to current TBMs. However i find it difficult to believe that tunneling costs will reach 1 million $/mile. The different types of strata, energy costs, use of advanced tech like lasers/supercapacitors in parts might help.