The first section became operational in 1863 when the Metropolitan Railway opened the world's first underground line between Paddington and Farringdon with wooden carriages and steam locomotives.

In 2009, the closed loop around the centre of London on the north side of the River Thames was broken at Edgware Road and extended west to become a spiral to Hammersmith.

In the same year, a select committee report recommended an "inner circle" of railway lines connecting the London termini that had been built or were under construction.

By May 1870, the District Railway had opened its line from West Brompton to Blackfriars via Gloucester Road and South Kensington, services being operated at first by the Metropolitan.

[8] Many breakdowns occurred, due to the unbalanced wear-and-tear inflicted upon the train and carriages caused by travelling in a single circular direction.

[8] The Midland Railway briefly ran a super outer circle from St Pancras to Earl's Court from 1878 to 1880.

[10] Experiments were carried out on the Earl's Court to High Street Kensington section, and a jointly-owned six-carriage train began passenger service in 1900.

After arbitration by the Board of Trade, the DC system was taken up, and the railways began electrifying the routes, using multiple-unit stock.

Both had open saloons; the Metropolitan trains with gated ends, the District B Stock with sliding doors in the middle of each car.

A fully electric service began on 24 September, initially with six-car trains, later reduced to four-car.

[13] The Metropolitan trains were soon modified to enclose the gated end[14] and eventually to add sliding doors in the middle.

[30] A day before a ban on drinking alcohol on public transport in London came into force, a party was held on 31 May 2008, mainly on the Circle line.

Thousands of people attended and 17 were arrested by police due to disorderly behaviour, eventually causing several stations to be closed.

[31] Prior to 13 December 2009, Circle line trains travelled in both directions around a simple loop with 27 stations and 12.89 miles (20.75 km) of track.

Much of the 2-mile-35-chain (3.9 km) double track railway from the Hammersmith terminus to Westbourne Park station is on a 20-foot (6.1 m) high brick viaduct.

Historically there has been difficulty in relaying the direction of travel a train is headed in a clear message: variations such as "eastbound" and "westbound", and "clockwise" and "anti-clockwise" can be ambiguous.

As passengers became more accustomed to digital devices, TfL considered stopping such announcements and now uses key stations along the route to describe a service (e.g. "via.

[46][47]From 1970 to 2014, services were provided using six-car C69 stock trains, each car having mostly transverse seating and four sets of double doors per side to minimise loading times.

[36] It was planned that a new signalling system would be used first on the sub-surface lines from the end of 2016,[57] but signalling contractor Bombardier was released from its contract by agreement in December 2013 amid heavy criticism of the procurement process[58] and London Underground subsequently awarded the contract for the project to Thales in August 2015.

[59] With the introduction of S7 Stock, the track, electrical supply, and signalling systems are being upgraded in a programme planned to increase peak-hour capacity on the line by 27 per cent by the end of 2023.

Owing to its historically circular nature, the line has generated many urban myths over the years, including a dead man travelling around undiscovered, a school or office using the service to save infrastructure costs and, as an April fool in the Independent, a new particle accelerator to coexist alongside passenger services.