Is there any study of an algorithm that relates population density to the potential traffic density and hence viability of potential new lines? If so, one could derive a map of underdeveloped regions from a rail traffic perspective.  It would also provide a guide as to when engineering problems created by mountains and seas are capable of being overcome in an economically viable way.  For instance, the Dublin/London route is one of, if not the busiest air route on the planet.  Could an undersea or oversea/bridge rail engineering solution ever become viable?

Index of Frank's Diaries