In just about every movie set in New York City in the 1970s and 80s there's an establishing shot with a graffiti-covered subway.
For city officials, train graffiti was a sign that they had lost control. So, starting in the early 70s, mayors of New York vowed to eradicate graffiti. First, Mayor John Lindsey formed the first anti-graffiti task force. He also re-classified graffiti from a nuisance, like littering or loitering, into a crime.
In 1984 David Gunn became President of the New York City Transit Authority.
Systemically, train line by train line, Gunn took the subways off the map for graffiti writers. While they were fixing it, they didn’t allow any graffiti on it. If graffiti artists “bombed” a train car, the MTA pulled it from the system. Even during rush hour.
May 12, 1989 was declared the official day of the city's victory over train graffiti.
But of course train graffiti has never stopped.
There is still subway graffiti—it just never leaves the train yards. Artists—many of them from abroad—paint subway cars knowing full well that they will get cleaned before they're ever seen by the public.