The triad will soon be a quad, if it isn't already. Who's going to stop a self tunneling nuclear tipped train that diggs X km below the surface? It will soon be

The triad will soon be a quad, if it isn't already. Who's going to stop a self tunneling nuclear tipped train that diggs X km below the surface? It will soon be a time critical arms race. Detonationing several megaton warheads deep enough may only leak a tiny amount of fallout, but with catastrophic earthquakes that could tumble even skyscrapers. This technology could alter the equation significantly as there would be no risk of nuclear winter, even if an entire continent gets carpeted.