| Map Colour: | Yellow |
| Year Operated: | 1884 |
| Year Opened: | 1863 |
| Year Named: | 1949 |
| Length: | 17 Miles (27.2km) |
| Journeys: | 74,000,000 / year |
| Stations: | Hammersmith Goldhawk Road Shepherd's Bush Market Wood Lane Latimer Road Ladbroke Grove Westbourne Park Royal Oak Paddington Edgware Road Baker Street Great Portland Street Euston Square King's Cross St. Pancras Farringdon Barbican Moorgate Liverpool Street Aldgate Tower Hill Monument Cannon Street Mansion House Blackfriars Temple Embankment Westminster St. James's Park Victoria Sloane Square South Kensington Gloucester Road High Street Kensington Notting Hill Gate Bayswater |
The London Tube's Circle Line appears on the London Tube Map as a yellow line and loops around London north of the Thames. It was originally authorised in 1853/54 when acts of Parliament allowed the Metropolitan Railway and the Metropolitan District Railway to build the first underground railway lines in the world in central London.
Work on the line stalled for various reasons before it's completion in 1884. Despite originally being operated using steam trains, electrification was introduced and took over completely in 1905.

Image from the Circle Line Article on Wikipedia.
The line took over from many parts of the Metropolitan line, and the success of the circular route led to the construction of further such routes within London:
Despite the success of the original circle, these extra routes did not perform as well and all three were eventually ended, though other services still continue on those lines today.