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.
Since the film's release, no #6 train has ever been scheduled to leave Pelham Bay Park Station at either 13:23 or 01:23 by the New York City Transit Authority. This was the practice for many years until the policy was discontinued. Trains are still not scheduled to leave the Pelham Bay Park Station at either 1.23 am or pm.
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.
Composer David Shire was asked to extend the end credits piece of score to allow for extra time/credits added in. His then wife, Talia Shire, suggested that he write in an adaptation of the theme that reflected a more romantic view of New York city, to contrast the gritty sounds used for the film's main score. David Shire adopted this approach in the extended passage.
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.