All scenes in the subway car were filmed in a studio mockup of IRT World's Fair Lo-V #5674. The producers contacted St. Louis Car Co. for original blueprints of the car and painstakingly reproduced it. Lights were mounted along the car exterior and illuminated sequentially to simulate a speed of 30 mph. The NYC Transit Authority refused to grant permission for filming on its property. Subway footage was filmed by concealing the cameras inside bags. Police became suspicious when they heard whirring sounds inside the bags.
The outdoor scenes of the El train were filmed on and around the Bronx section of the Third Ave El, which closed in early 1973 and was torn down shortly after .
This movie was Thelma Ritter's second to last before she suddenly died from a heart attack in 1969 just nine days before her sixty-seventh birthday.
A UK cinema release was rejected outright by the BBFC twice in January 1968. It has not been resubmitted since.