Answer:
The London Underground (Tube) serves the actual City of London, known affectionately as "The Square Mile" and Greater London. Greater London is made up of cities or 'boroughs' and are in the Coulnty of London. The tube serves many outlying cities as it stretches from the main 'circle' to places as far as Heathrow Airport but I do not believe there are any journeys more than an hour from 'Central London'.