To warn of the space between the train & the platform, London subway riders are told to "mind" this
The phrase was first introduced 
in 1969 on the London Underground in the ... On London's Metropolitan line, a 
gap has been created between the train... [DOC] [TLE] Phil Sayer, 62; cautioned London subway riders to 'mind the gap' [PAR] Apr 17, 2016 ... Mr. Sayer's voice told riders on the London Underground to please mind the gap 
... to please mind the gap between the platforms and train caras.