According to Wikipedia's stats, the population of New York declined slightly between 1950-1960, recovered 1960-1970, and then declined again, more sharply, 1970

According to Wikipedia's stats, the population of New York declined slightly between 1950-1960, recovered 1960-1970, and then declined again, more sharply, 1970-1980. The big growth years started after 1980 - i.e. around the same time their ridership tracking began. Correspondingly, the cultural legacy of New York as it started the 80's was one of a city in disarray with trashed streets and little public order or prospects for a future - great material for gritty films about crime and societal breakdown. The economic situation and mood started changing over the course of the decade, and the image of "Disneyfied New York" finally emerged in the 90's under Guliani's mayorship. So, going back to the original question, New York went through a rough period following WWII, and that combined with a broad switch to cars accounts for changes in ridership.