The Circle line shares almost it's entire route with three other lines, the District line, Hammersmith and City line and the Metropolitan line. There are two sections of track, that the line uses exclusively, these are between High Street Kensington and Gloucester Road and between Aldgate and Minories Junction (east of Tower Hill). It takes an hour to travel the entire route of the Circle line, which serves most of London's railway termini.
Creating the 'inner circuit'
The constriction of the 'inner circuit' railway was the result of Parliamentary committees in 1863 and 1864, which proposed the construction of a connection to the Metropolitan Railway, at both ends, to connect the mainline railway termini on the northern side of the Thames, at Victoria, Charring Cross, Blackfriars and Cannon Street.
The initial proposal was to extend the railway from Moorgate to Cannon Street however this was amended to allow the construction of three additional termini at Liverpool Street, Broad Street and Fenchurch Street. However this caused a complex layout at Aldgate that would be less desirable for local traffic and greatly increase the cost of construction due to the costs of developing in the City of London.
The Metropolitan Railway obtained powers on 29 July 1864, to extend the railway to South Kensington, in the west, and Tower Hill, in the east. Also on 29 July 1864, the Parliamentary committee encourages the creation of a secondary company called the Metropolitan District Railway. The concept was to merge the two companies after construction however this was blocked by the shareholders of the Metropolitan Railway because the Metropolitan District Railway was less profitable. However the construction of the railway was entered as a single contract.
The first section of the new 'inner circle' was opened by the Metropolitan between Paddington and Gloster Road (opened as Brompton) on 1 October 1868. With the second section quickly being opened by the Metropolitan District Railway between South Kensington and Westminster on 25 December 1868. The next section to be opened, in 1871, allowed services to run from Manson House to Moorgate ia South Kensington and Paddington. Between High Street Kensington and South Kensington both the Metropolitan Railway and Metropolitan District Railway operated separate tracks.
The Metropolitan District Railway quickly ran into financial difficulties and planned to abandon it's route to Tower Hill. Businesses in the City of London where not willing to allow this to happen, and formed the Inner Circle Completion Railway Company in 1874, causing the Metropolitan District Railway and Metropolitan Railway to negotiate.
The Metropolitan Railway began a further extension towards Aldgate, opening in two stages, from Moorgate to Liverpool Street opening on 12 July 1875 and Aldgate on 18 November 1876.
The Metropolitan Railway brought out the Inner Circle Completion Railway Company in 1879 and work to compleate the southern end of the circle quickly resumed.
A final extension to the Metropolitan Railway to a temporary station at the Tower of London was opened in 1882. The Metropolitan District Railway opened it's final extension to Whitechapel on 6 October 1884. With a full circle service beginning on the 6 October 1884, with the temporary station at Tower of London being replaced by a permanent station and permanently closing on 12 October 1884.
Upon opening, there was a fierce rivalry between the two companies. Each company ran services in a single direction, the Metropolitan Railway ran 'outer rail' (clockwise) services and the Metropolitan District Railway operated services on the 'inner rail' (counter clockwise).
Initially both companies provided the same number of services, however, on 10 November 1884 with a service alignment the Metropolitan Railway provided all 'outer rail' (clockwise). In late 1907 and the whole of 1908 the Metropolitan Railway provided all services.
Electrifying the 'inner circuit'
When the railway opened the services where provided by wooden card hauled by steam locomotives, however, with increasing competition with the new electric deep-level underground lines the railways became unpopular with passengers. As a result, a joint venture to experiment with electric trains began between Earls Court and High Street Kensington in 1900, with a single 6 car service.
The District Railway needed to refinance in 1901 and began to look for an investor. They where invested and amalgamated into the Underground Electric Railways of London, this caused issues as the District and Metropolitan Railways where using different electric systems. The Metropolitan Railway, later decided to convert their rolling stock to be compatible with the District system.
A full electric service began on 24 September 1905, initially this ran as a 6 car service but this was quickly reduced to a 4 car service. A 5 car service began in 1918.
The District Railway began to operate regular regular services to Edgware Road, with the Metropolitan taking over the operation of the circle apart from 3 services on a Sunday, this arrangement lasted until 1990.
Circle line today
On the 1 July 1933, the District Railway and Metropolitan Railway where amalgamated by the London Transport Passenger Board. The original electric Metropolitan Railway rolling stock was refurbished at Acton Works, painted red and cream and 18, 5 car trains entered service, these remained in service until 1947.
The 1933 Henry Beck tube map shows the northern section from High Street Kensington to Mark Lane as a part of the Metropolitan Railway and the southern section as a part of the District Railway. The two separate colours where amalgamated in 1947, with the line obtaining its separate identity in 1949.
Since 13 December 2009, the Circle line has operated though to Hammersmith, this was introduced to decrease the service interval and follows the Hammersmith and City tracks. The typical route for a service is Hammersmith to Edgware Road, via Central London, returning over the same route. The route spans 27km (17 miles) and serves 35 stations.
