I gather the very reason it's the second-busiest air route in the world is that it's just far enough below the margin at which it becomes economically feasible

I gather the very reason it's the second-busiest air route in the world is that it's just far enough below the margin at which it becomes economically feasible to build fast rail (for now). As a comparison, London and Paris, which are of comparable proximity and economic importance to one-another, have much bigger populations, and thus fast rail is justifiable, so the two cities don't rank highly on the busiest-air-routes list.