Most of the world's busiest systems have at least a line or several which approaches 90 second intervals at peak hours. Moscow, Paris, London, even NYC (and I w

Most of the world's busiest systems have at least a line or several which approaches 90 second intervals at peak hours. Moscow, Paris, London, even NYC (and I would have to assume many Asian systems I'm not as familiar with as well). 30-40 trains an hour is the gold standard, but obviously only makes sense if there is enough ridership.