One point people seem to be ignoring is that Elon mentions on multiple occasions that he plans to build in 3D, i.e. not just one layer of tunnel in one directio

One point people seem to be ignoring is that Elon mentions on multiple occasions that he plans to build in 3D, i.e. not just one layer of tunnel in one direction but multiple tunnels at multiple depths in multiple directions i.e. to get your 36k pax/hr will require 4 layers.