Established in 1935, the New York City Transit Police Department was responsible for the protection of New York subway lines for 60 years. In 1949 the department was officially divorced from NYPD, but was eventually fully integrated in 1995.
One of the more controversial roles of the Transit Police has been its defense of the subway system from what politicians see as defacement. Graffiti art, one of the four elements of hip hop, was begun in the NYC subway system and was very prominent by the mid-1980s. The city establishment took a hard line in response to this underground movement. The Transit Police and specifically a new unit called the Vandal Squad began to fine and arrest graffiti artists. They also made a policy to remove any work of graffiti within 24 hours of its creation.
By the end of the '80s the Transit Police had effectively solved the "problem" of graffiti in the subway system. To this day the Vandal Squad exists to prevent "the look of disorder" from over-running the city.