The three SST tracks available to Concorde over the North Atlantic are fixed; whilst the regular NAT tracks are changed daily, and so may vary considerably in position, depending on the prevailing winds.
Thus the NAT tracks could quite easily be, and sometimes are, right underneath the SST tracks, for part or all of the crossing.
As a matter of trivia, Concorde
does fly supersonic over land on one route - although it's not a heavily populated area.
A free drink for the first person, who is attending tomorrow's GatBash, who posts the correct route and the land mass concerned here!
