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"Mind the gap" is a warning to train passengers to remind them of the sometimes significant gap between the train door and the station platform. It was first introduced in 1969 by the London Underground rapid transit system.
Some station platforms on the London Underground are curved. Since the cars are straight, the distance from the platform to the car at certain points is greater than normal, and the phrase "MIND THE GAP" is painted on the edge of such platforms. A recorded announcement is also played whenever a train arrives at a station, consisting of "Mind the gap" repeated several times, followed by "Stand clear of the doors" as the train is about to depart.