Some station platforms on the London Underground railway are curved. Since the carriages are straight, the distance from the platform to the carriage is greater than normal, and at such stations the words "MIND THE GAP" are painted on the edge of the platform. A recorded announcement is also played whenever a train arrives at a station, consisting of "Mind the Gap" repeated over and over, followed by "Stand clear of the doors, please." as the train is about to depart. The Central Line platforms at Bank and the Bakerloo Line platforms at Piccadilly Circus are some of the more extreme examples.
Deep level tube trains have a floor height around 20cms less than cut-and-cover trains. Where these trains share platforms, for example some Piccadilly Line (deep level) and District Line (cut-and-cover) stations the platform is built as a compromise between the two. Therefore the "mind the gap" warning is used in this situation as well.
The phrase is well enough known for London Underground to print it on T-Shirts sold to the public.
The phrase also inspired a song of the same name by Judge Dread.
The phrase is also used by other transit systems, notably the Toronto Transit Commission who have their own logo posted prominently in all train stations: