In a TVO (Ontario, Canada) interview, the producer said that this film did terrific box office in New York, Toronto, London and Paris--all cities with subways--but was considered a flop in the rest of the world.
One of Robert Shaw's favorite pastimes during breaks in filming was playing ping-pong on tables set up on the station's concourse. He reportedly badly beat all challengers.
Lee Wallace, who plays the mayor of New York in this film, bears a striking resemblance to the real mayor Ed Koch. Many reviewers thought he was playing Koch, but Koch would not be the mayor for another four years.
The $1M in the number and denomination of bills as ordered by the hijackers would weigh 33 US pounds (15 kg).
The reason for the large gap between the subway cars and the platform is that a large portion of the underground scenes was filmed on the unused Court St. stub of the IND subway, which uses wider cars than the IRT cars used in the film.