The Northern line began as two separate railways the City and South London Railway (CSLR) and Charing Cross Euston and Hampstead Railway (CCEHR). The City and South London Railway (CSLR) began its services on 4 November 1890 and was opened by Prince Edward Prince of Wales (later Edward VII), the railway spanned 5.52km (3.26 miles) and had 6 stations. The Charing Cross Euston and Hampstead Railway (CCEHR) began operating on 2 June 1907 and was opened by David Lloyd George, President to the Board of Trade and had 16 stations. The railways merged during the 1920s with the completion of several extensions and works to improve the railways compatibility. The 'Northern line' became the railways name on 28 August 1937 to reflect a gigantic extension to the line called the 'Northern Heights'. The line is the busiest line on the London Underground network, calling at 50 stations over 58km (36 miles). The line is also due to receive an extension opening in 2020 from Kennington to Battersea Power Station.

City and South London railway

City and South London Railway (CSLR) was the first deep-level underground railway to open in the world, further to, being the first major railway to use electric traction. However, it was proposed by private bill during November 1883. The bill was to construct a deep-level underground railway between King William Street in Central London to Elephant and Castle. The railway would be constructed using a tunnel-shield or a segmented cast-iron tube and would comprise of twin tunnels 3.1m (10ft 2in) in diameter running a total of 2.01km (1.25 miles). Royal Assent was obtained on the 28 July 1884 with the bill being called the City of London and Southwark Subway Act 1884.

The Railway proposed a new law, to allow for a 2.82km (1.75 mile) extension to the existing planned railway and to construct the tunnels in a larger 3.2m (10ft 6in) diameter. This received Royal Assent on 12 July 1887 and became the City of London and Southwark Subway (Kennington Extensions &C) Act 1887. The railway proposed another act, before it opened in 1883, to allow for a further extension to South Clapham and to change the name of the railway, this passed on 25 July 1890 as the City and South London Railway 1890.

The railway was officially opened on 4 November 1890 by Price Edward Price of Wales (later Edward VII) with the first passenger services beginning on 18 December 1890. The railway ran for 5.52km (3.26 miles) and called at 6 stations: Stockwell, The Oval (Oval today), Kennington, Elephant and Castle, Borough and King William Street. The original service consisted of an engine and three carriages, the carriages could accommodate 32 passengers each and had longitudinal high-back bench seating with narrow windows along the side of the carriage. The carriages also had gate-men who rode on the carrage platforms, they called out the station names to passengers and operated the lattice gates at the ends of the cars, while the train was at a station.

The claustrophobic interiors of the trains earned the railway the nickname 'padded cell'. When the railway began operating it did not use paper tickets nor a class system, fares where collected on a turntable and the fare was a flat 2d. Despite the claustrophobic conditions, the railway saw 5.1 million passengers in 1891, although it was competing with bus and tram operators. To alleviate this overcrowding the rolling stock was enlarged.

The railway intended to extend even before it was opened to the public, a plan to extend to Islington was rejected by the government. The proposal didn't provide a direct connection from the existing railway and would be accessed by passengers disembarking the trains at King William Street and continuing though underground passageways to another platform to continue their journey. The railway re-proposed this extension with a few amendments in November 1891, the new extension would bypass the new problematic King William Street with a new pair of tunnels. The railway would also introduce a new station near Borough to allow for interchange between the London, Brighton and South Coast Railway (LB&SCR) and London Eastern and Chatham Railway (LE&CR) at London Bridge, the tunnel would then pass east of the existing mainline station and head northwards though the City of London to Angel. The railway would also be extended southwards to Clapham, as another plan proposed to parliament recommended, these plans recived Royal Assent on 24 August 1893 as the City and South London Railway Act 1893.

Construction of the new extensions was delayed while the railway raised the funds and finalised its plans. The railway proposed various legislation to allow for the construction as the company needed to obtain additional permissions and approvals to continue. The City and South London Act 1895 which allowed for the construction of a new tunnel approach to King William Street received Royal Assent on 14 April 1895. The City and South London Act 1896 allowed for more time to construct the extensions in the 1893 act and made various changes to the new station being constructed at Bank received Royal Assent on 14 August 1896. The City and South London Act 1898 added sidings to the new southern extension at Clapham Common and to sell the station at King William Street and approach tunnels for the new City and Bixton Railway received Royal Assent on 23 May 1898.

The newly constructed tunnels at King William Street allowed for a new track layout with a single platform wedged between the two running tracks on each side opened temporarily in 1895 while the railway raised money for its planned extensions. The southern extension opened on 3 June 1900 to Clapham Common. King William Street station later closed on 25 November 1900, with the opening of the northern extension to Moorgate Street. The railway received permission to enlarge the station at Angel and the tunnel diameter to 9.2m (30ft) on 25 May 1900 when the City and South London Railway Act 1900 received Royal Assent. The remainder of the northern extension to Angel opened on 17 November 1901.

The railway ran into financial difficulties, however remained in high demand and was accused of extravagantly abandoning King William Street station. In a bid to improve the financial standing and reputation of the company, the railway proposed an extension to Euston under the Islington and Euston Railway. The newly constructed line would operate from Angel to Euston via Kings Cross and St Pancras. The extension was considered by the government alongside a number of other proposals which where influenced by the opening of the Central London Railway in 1900. The government preliminary agreed with the proposal and gave its permission, however its permission was too late for the current session in Parliament and never became a law. The railway decided to resubmit its application in the next Parliamentary session, with an amendment to for the City and South London Railway (CSLR) to take over the operations of the railway upon opening, however, the proposal was seen as a threat to the Metropolitan Railway and the proposals where rejected.

The railway decided to re-enter an amended bill in November 1902, to allow for the railway to be extended to Euston with interchange with the Hampsted Euston and Charring Cross Railway and a new station at King William Street with a pedestrian subway linking the new City and South London Railway (CSLR) Bank station and the District Railways Monument station. A third pair of tunnels would be constructed underneath the Thames to connect the abandoned tunnels north of Borough and to construct the approved connections at London Bridge and Oval. These proposals received Royal Assent on 11 August 1903 as the City and South London Railway Act 1903, the extension opened on 12 May 1907.

The Underground Electric Railway Companies of London (UECL) and other underground railway companies began to work together with joint marketing and ticketing from 1907, with the distinctive 'UndergrounD' logo being paced outside Central London stations from 1908.

The City and South London Railway proposed to enlarge the diameter of its tunnels to increase capacity in 1912. Along with the Underground Electric Railway Companies of London (UECL) proposing to extend the Charing Cross Euston and Hampstead Railway (CCEHR) and City and South London Railway (CSLR) to Camden Town to enable the railways to share stock and the lines to be operated as a single railway. Both of these plans received Royal Assent on 15 August 1913 as the City and South London Act 1913 and London Electric Railway Act 1913.

The City and South London Railway continued to struggle financially and on 1 January 1913 the railway was purchased by the Underground Electric Railway Companies of London (UECL), who paid 2 shares for the original City and South London Railway rolling stock, the discount showing how much the company struggled.

Charring Cross, Euston and Hampstead railway

The Charing Cross Euston and Hampstead Railway (CCEHR) was proposed by a private bill during November 1819, the railway was originally planned to be called the Hampsted, St Pancras and Charing Cross railway. The railway would be underground from Heath Street in Hampsted to Charing Cross, the railway would pass beneath Hampsted High Street, Roslyn Hill, Haverstock Hill, Chalk Farm and Camden Town along its northern alignment. The railway would proceed southwards and pass beneath Camden High Street, Hampsted Road, Tottenham Court Road, Charing Cross Road and King William Street to Angar in the Strand. A further branch was proposed underneath Drummand Street to allow the railway to construct stations at Euston, Kings Cross and St Pancras. Stations would be constructed at Hampsted, Belsize Park, Chalk Farm, Camden Town, Seymour Street (Evershot Street today), Euston Road, Tottenham Court Road, Oxford Street, Angar Street and Euston and Kings Cross. The plan also included the construction of an electric power station on Chalk Farm Road.

This proposal was inspired by the large passenger numbers that used the City and South London Railway (CSLR). A joint committee was set up by the government to consider this proposal and a further three that where submitted. The committee took evidence and made reconsiderations on various aspects of deep-level tunnel construction, the committee gave advise primarily on the tunnel diameter, method of traction and granting wayness. The southern central London route was granted preliminary permission, however the committee left the northern section beyond Euston to normal parliamentary consideration. The route was finally approved and obtained Royal Assent on 24 August 1893 as the Charing Cross, Euston and Hampstead Railway Act 1893.

Although the railway obtained permission to construct it hadn't raised capital. This was not the only railway with this problem, there where four other railways bidding for investors; the Baker Street and Waterloo Railway, Great Northern and City Railway, Waterloo and City Railway and Central London Railway. The only railway which managed to obtain investors quickly was the Waterloo and City Railway as its shares where guaranteed by the South Western Railway (SWR).

The railway got a share offer in 1894, however this was unsuccessful. By December 1899, the railway had sold 451 shares to 8 investors for £10 each, the railway had a total of 177600 shares it needed to sell before it could begin its construction. With the slow uptake by investors, the railway asked the government for a series of time extensions, the first of these was to allow additional time for compulsory purchase orders in 1893. Four further Acts where passed to provide further time extensions to key milestone deadlines, these where passed in 1897, 1898, 1900 and 1902 all being called the Charing Cross, Euston and Hampsted Railway Act.

A contractor for the construction of the railway was selected in 1897, however the contractor could not begin construction because the company didn't have the relevant capital.

During September 1900, a private financier called Charles Yerkes bought the railway for £100,000. This was one acquisition with this person buying a series of unbuilt underground railways in London and the operational but struggling Metropolitan District Railway. The Underground Electric Railway Companies of London (UECL) was established to manage the railways from Charles Yerkes, to construct these railways and electrify the Metropolitan District Railway. The company managed to raise £18 million (the equivalent to £1.74 billion today) for all of its projects.

A bill was presented to government on 24 November 1894, to allow for the purchase of additional land to construct stations at Charing Cross, Oxford Street, Euston and Camden Town; this received Royal Assent on 20 July 1895. A further bill was presented on 23 November 1898 to change the southern terminus of the railway to underneath Craven Street in the south of the Strand area, this received Royal Assent on 25 July 1898. A bill proposing a new route to extend the railway from Camden Town to Kenish Town which would allow for an interchange with the Metropolitan Railway station, a surface depot to be construced at Highgate Road, to extend the railway to connect its Euston route south of Camden Town and to allow for a new site at Cranbourn Street (later Leicester Square station). This received Royal Assent on 9 August 1899 as Charing Cross, Euston and Hampstead Act 1899.

The railway proposed a series of wide ranging changes to its current alignments and presented two bills before parliament on 23 November 1900. The first, proposed constructing the railway from Hampstead to Golders Green and the purchase of additional land to construct a depot. The second, proposed constructing the railway extending northwards beyond Kentish Town to Brecknock Road, Archway Tavern, Archway Road and Highgate. A further extension to the south was also proposed extending the railway from Charing Cross to Parliament Square, Artilley Road and Victoria Station.

The first proposal was controversial, the railway would be constructed into open countryside and farmland; Charles Yerkes was accused of planning to sell the land at a profit once the railway had been constructed. However, both bills where presented to parliament to set-up a joint committee to review the bills. The committee produced a report too late for the parliamentary session and the bills where resubmitted. A third bill was added, when the bills where resubmitted, this proposed a short extension southwards from Charing Cross to the Victoria Embankment to allow for interchange with the District Railway, which already had a station there. The bills where re-examined by committee and the extensions northwards to Highgate and southwards to Victoria where quickly stuck out.

There was a large public opposition to the construction of tunnels underneath Hampstead Heath, with a local opposition group saying that the trains would drain the sub-soil and the vibrations from passing trains would damage the trees. The tunnels would be excavated 61m (200ft) below the surface, and the objections where deemed obscured.

A second railway, the Hampstead and Edgware Railway (H&ER), had planned to construct tunnels underneath Hampstead Heath decided to share its service with the railway. Initially the local authority refused to allow the tunnels to be constructed, but eventually they gave their consent with the condition that another station is constructed on the northern edge of the Heath to serve a new residential estate planned for the area. Parliament was convinced that there would be no damage to Hampstead Heath and the bill received Royal Assent to both railways as the Charing Cross, Euston and Hampstead Railway 1902 and the Edgware and Hampstead Railway Act 1902.

The sites where demolished and preliminary works began during July 1902.

A further bill was presented to parliament on 21 November 1902, this was to provide compulsory purchase orders for the new station buildings, a formal agreement for the take over of the Edgware and Hampstead Railway when it was constructed and a formal abandonment of the redundant Kentish Town branch to Highgate Road. Royal Assent was achieved on 21 July 1903 as the Charing Cross, Euston and Hampstead Railway Act 1903.

North End station was sold to a group of conservationists who opposed the extension to the railway beneath Hampstead Heath in 1904, this bought a reduction to the project number of passengers using the station. However, the work continued although at a slower pace, the platform tunnels and passenger circulation tunnels where compleated before the station was abandoned in 1906, there where no lift or stair shafts constructed neither had a station building.

The tunnelling was competed during December 1905, the construction of station buildings and fitting the tunnels with equipment became the priority. The planned power station at Chalk Fram Road was no longer needed because the railway would be powered by the Underground Electric Railway Companies of London (UECL) power station at Lots Road. The short tunnels between Charing Cross and the Victoria Embankment had not been constructed, on opening Charing Cross was the termini of the railway.

The railway was opened by David Lloyd George, President to the Board of Trade on 2 June 1907. To celebrate the public got to ride the railway free of charge for the day. The railways name was abbreviated from its opening day, the railway was commonly know as the Hampstead Tube or the Hampstead Railway, these where shown on contemporary maps and station buildings.

The service began using carriages and rolling stock developed for the Underground Electric Railway Companies of London (UECL) by the American Car Foundry and was the same stock that was used on the opening of the Baker Street and Waterloo Railway and Great Northern Piccadilly and Brompton Railway. Passengers would board the carriages by a folding lattice gate located at each end of the carriage and where operated by gate-men who travelled on the outside platforms and announced station names to the passengers.

The railway was used by 25 million passengers, in its first twelve months, half of the predicted levels before the railway opened. This was the picture across the network, the introduction of electric trams and motor buses saw the removal of slower horse-drawn carriages increased competition between the different modes. This lead to financial problems for the railways because they didn‘t make as much money as they planned and struggled to pay back loans.

To improve the finances of the company, the Underground Electric Railway Companies of London (UECL) in 1907 began to work with the other underground railways in London. The companies, but the Waterloo and City Railway, joined together to provide a single image to the underground network. This included joint ticketing, publicity and marketing; the first signs with the famous ‘UndergrounD‘ logo began to appear in central London in 1908.

The Underground Electric Railway Companies of London (UECL) also began to reduce the administration of the company. The railway proposed a bill during November 1910 to merge the existing Charing Cross Euston and Hampstead, Great Northern Piccadilly and Brompton, and Baker Street and Waterloo railways into a single entity. The railways would keep their distinctive unique identities, however the merger would streamline the management process. The bill passed, and obtained Royal Assent on 26 July 1910 as the London Electric Railway Act 1910.

The railway proposed to revive the Victoria Embankment extension during November 1910. The railway would be extended by a single tunnel running underneath the Thames and connect to the existing tunnel infrastructure allowing for trains to run in one direction. The railway would also construct a station, which would have interchange with the Baker Street and Waterloo railway and the Metropolitan District Railway. The proposal received Royal Assent on 21 June 1911 as the London Electric Railway Act 1911. The loop was quickly constructed with a large excavation at the Metropolitan District Railway station and the new station connected to the subsurface station. The station opened to the public on 6 April 1914.

The Edgware and Hampstead railway was still looking to finance its project. The Charing Cross Euston and Hampstead railway decided to revive these plans and propose a further extension to the railway with the Watford and Edgware Railway (W&ER). This would extend the railway as far as Watford via Edgware. The Watford and Edgware Railway (W&ER) would take over the Hampstead and Edgware railway and construct a railway from Golders Green to Edgware, this received Royal Assent as the Watford and Edgware Railway Act 1906. However, the company struggled to raise enough capital and in 1906 proposed constructing the railway as a light-railway, this was rejected by the government. Once the powers lapsed for the construction of the extension the powers where passed back to the Charing Cross Euston and Hampstead railway.

The Edgware and Hampstead Railway plans remained in existence, the railway passed a series of legislation in 1905, 1909 and 1912 to provide the relevant time extensions approve changes to the planned route and to gain permissions to construct a viaduct, a short tunnel and the relevant road closures and openings. The Charing Cross Euston and Hampstead railway also formalised its plans to take over the railway once the railway extension had been completed. This was formalised in the London Electric Railway Act 1912.

The relevant permissions where granted to construct the railway, however the outbreak of the First World War prevented its construction. The extensions where guaranteed in annual legislation between 1916 and 1922, with the final date for compulsory purchases being given as 7 August 1924. However, the permissions where not retained and the railway could not rise the capital it requited, further to increasing construction costs due to the war and the projected number of customers being revised where the railway would be unable to repay its loans.

The railway struggled to finance its extension until the government passed the Trade Facilities Act 1921, which guaranteed public works by the treasury underwriting their loans, this was planned to reduce unemployment. After these assurances where given, the Underground Electric Railway Companies of London (UECL) managed to raise the required to construct the extension and the works began ceremoniously on 12 June 1922 with Lord Ashfield at Golders Green.

The extension mostly crossed open countryside and farmland, therefore the railway could be constructed over the surface with the exception of a viaduct over Brent Valley and a short tunnel at The Hdye, Hendon. The extension opened from Golders Green to Hendon Central on 19 November 1923. The remainder of the extension opened to Edgware on 18 August 1924.

The London Electric Railway announced on 21 November 1922 that it would enter a bill to propose to extend the railway southwards to Kennington via Waterloo to provide an interchange with the City and South London Railway (CSLR) station at Kennington. This bill was passed as the London Electric Railway Act on 2 August 1923. Construction began to rebuild beneath the existing railway terminus to extend the tunnels under the Thames to Waterloo. Kennington had two new platforms constructed for interchange, outside the station the tunnels joined the City and South London Railway (CSLR) tunnel. The station opened on 13 September 1926, the same time as the City and South London Railway (CSLR) extension to Morden.

The Underground Electric Railway Companies of London (UECL) purchased the City and South London Railway (CSLR) in 1913. Camden Town was redesigned and a connection constructed to allow services to share tracks from 1924. The opening of these connections at Camden Town and Kennington allowed the railways to be operated as a single railway, and their services where integrated.

Creating the Northern line today

During the 1920s a series of works on the City and South London Railway (CSLR) and Charing Cross Euston and Hampstead Railway (CCEHR) made connecting the railways easier. The first tunnel was to connect the City and South London Railway (CSLR) station at Euston to the Charing Cross Euston and Hampstead Railway (CCEHR) station at Camden Town, this was initially planned in 1912 however it was delayed by the outbreak of the First World War. The second tunnel was between the Charing Cross Euston and Hampstead Railway (CCEHR) station at Embankment and the City and South London Railway (CSLR) station at Kennington, this allowed for an intermediate station to be constructed at Waterloo providing an interchange with the Baker Street and Waterloo railway (BSWR)Baker Street and Waterloo railway
A railway that has been absolved into the Bakerloo line. and mainline station.

The railways operated using different tunnel diameters, therefore the City and South London Railway (CSLR) tunnels where expanded to the diameter used for the Charing Cross Euston and Hampstead Railway (CCEHR).

A plan to extend to Edgware had been proposed since 1901 with the Edgware and Hampstead railway being bought by the Underground Electric Railway Companies of London (UECL) in 1912. The plan was to extend the railway from Golders Green in two stages, the first to Hendon Central opening on 19 November 1923 then Edgware opening on 18 August 1924. The extension included five intermediate stations designed by the architect Stanley Heaps, this lead to a rapid growth in developments in the following years.

The extension to Morden was more challenging, with the construction of a tunnel from Clapham Common to north of Morden Station. The station was constructed in a cutting and the railway runs underneath the station forecourt and a main road to Morden Depot. Initially the railway was planned to be constructed to Sutton over the unbuilt Wimbledon and Sutton Railway, however an agreement with the Southeastern Railway prevented this. This extension induced seven intermediate stations designed by the architect Charles Holden and opened in 1926.

The railway became known as the Morden - Edgware line, however there where alternative names floated based on the fashionable consolidation at the time, such as: Edgmor, Mordenware, Medgway, Edgmorden. The name ‘Northern line‘ was officially adopted on 28 August 1937 to reflect the new Northern Heights planned extension.

The railways where nationalised on 1 July 1933 by the London Transport Passenger Board (LTPB), who took over the operation of the underground railways, buses and trams in the London area. The board operated the Great Northern and City Railway, a former subsidiary of the Metropolitan Railway, as a part of the Northern line in preparation of the Northern Heights extension This became known as the Northern City line and remained an independent railway to the Northern line, never being connected.

An ambitious plan called the New Works Programme was announced during June 1935. The plan included plans to change the Northern line, by integrating the London and North Eastern Railway (LNER) Highgate station through the Northern Heights extension with railways that where constructed between 1860 and 1870 by the Edgware Highgate and London Railway (EH&LR). The plan included an extension from Edgware to Brockley Hill, Elstree South and Bushley Heath with a new depot at Aldenham the route would follow the unbuilt Watford and Edgware Railway (W&ER) rights which where obtained to extend towards Watford.

This project would envolve the electrification of the surface lines, the doubling of the track between Finchley and Edgware, and the construction of three linking sections. The first would link the existing Northern and City line to Finsbury Park, the second, an extension from Archway to a new station at the London and North Eastern Railway (LNER) station at East Finchley and a new deep-level station at Highgate, and finally, a short diversion from the London and North Eastern Railway (LNER) station at Edgware to the underground station.

Work began during the 1930s, by the outbreak the work had moved quite quickly. The tunnelling beneath the original Highgate station had been completed along with the rebuilding of East Finchley, public service starting on 3 July 1939, however Highgate station did not open at this time. The electrification between East Finchley and High Barnet with a tube service stating on 14 April 1940. A deep-level station at Highgate opened on 19 January 1941. With the line between Finchley and Mill Hill East, using the original London and North Eastern Railway (LNER). line to Edgware, being opened on 18 May 1941; including over the Dollis Hill Viaduct. A depot at Aldenham had been completed and was converted to construct Halifax bombers. These works where suspended in 1939 to assist with the war effort.

Prelimary works had have been started on a viaduct and tunnel in Bushley, however these had not been complicated After the war the area north of Edgware was declared green belt, which prevented the anticipated development and diminishing demand the plan was officially dropped on 9 February 1954. The depot at Alenham was converted for use with buses.

The service serving High Barnet proved to be sufficient to the City and the West End of London. However, the Alexandra Palace branch, still using steam haulage to Kings Cross via Highgate had a low demand and was closed in 1954. A contract was offered to electrify the Epping to Ongar branch on the Central line in 1957, this prompted a local pressure group to propose constructing the line as a light railway. However, there was no movement and today the route is enjoyed by many presentation and cyclists.

The Northern line today

The Northern City line, commonly refereed to as the Highbury branch, was transferred to British Railways during 1975 and is currently served by mainline railway services.

The Northern line became increasingly unreliable in the 1980s and 1990s, the line was nicknamed by the local press as the ‘misery line‘, until the introduction of the 1995 stock in the late 1990s.

There have been several instances of incidents the railway, in 2003 a train derailed outside Camden Town station. No person was injured during this incident however, there was significant damage sustained to the points, signals and the carriages. The damage caused the line to operate with Edgware branch services operating via Bank and High Barnet services operating via Charing Cross. A full service was restored on 7 March 2004. A report into this incident stated that there was poor track geometry, there was significant additional friction which caused striations (scratches) on the new equipment which led to the rear wheel leaving the track and derailing the train. The incedent occured on a section of track which had a tight bend and tunnel bore, the solution was to raise one track higher than the relitave track on the other side.

The London Underground network was a victim in the 7 July 2005 terrorist attack on London. The service on the Northern line had been suspended due to a defective train. There where three trains on the Circle and Piccadilly lines which where bombed, with the bomber who intended to detonate their device on the Northern line was unable to and boarded a bus, which they later blew up.

The service on the Northern line was completely suspended on 13 October 2005, this was because of maintenance issues on the emergancy breaking system on the entire fleet. A full service was restored on 18 October 2005.

The High Barnet branch was suspended for two non-consecutive weekends between East Finchley and Camden Town, starting on 13 October 2005. The Edgware branch quickly followed; this was to allow for the redevelopment of stations, replace track and signals. This included a refurbishment of the stations between West Finchley and Camden Town on the High Barnet branch.

The service between Finchley Central and Mill Hill East was changed during October 2006, to allow for a shuttle to operate between the two stations and a peak time service to be withdrawn except for during the weekend.

A defective rail grinding train operating on the High Barnet branch dislodged without a driver and proceeded down the line for approximately 13 minuets eventually stopping on a steep gradient outside of Warren Street station on 13 August 2010. The train was defective and towed to a depot on the line, the services where disrupted, with the Charing Cross branch being closed and all peak services operating via Bank, some services where turned into ‘non-stopping‘ to ensure the railway remained safe for the public to use.

The Northern line was upgraded to support Automatic Train Operation (ATO) and uses the same system that is currently used on the Jubilee line and Docklands Light Railway. The inital plan was to upgrade the line, in succession from the completion of the Jubilee line upgrade. However the upgrade did not finish until Sprint 2011, therefore the plan to introduce Automatic Train Operation (ATO) the line prior to the 2012 London Olymics was revised. The implementation of Automatic Train Operation (ATO) on the Northern line began on 26 February 2013, with the section between West Finchley and High Barnet. The project was completed on 1 June 2014 with the section between Chalk Farm and Edgware being upgraded.

There have been plans to split the Northern line, to their original operating routes, this is a particular aspiration for Transport for London. To introduce this concept, Camden Town station would have to be reconstructed because it would be unable to cope with the numbers of passengers who would interchange, however the service would be improved with a split. The line currently supports a maximum of 24 trains per hour over the branches, after a split the segregated lines would allow for a maximum of 30 to 36 trains per hour.

Transport for London introduced a plan to reconstruct Camden Town, the plan included a plan to demolish the existing surface buildings; this was rejected by the local authority in 2005. Two further proposals have been put forward in 2013 and 2015 to address the underground interchange rather than previously addressing general entrance and exiting issues. If these proposals are approved, work should begin between 2018 and 2024.

The line is currently running a segregated service during normal operations, with services running via Bank to Morden, and Charing Cross to Kennington. This allows for an increase in capacity because there is no need to co-ordinate services southbound from the Charing Cross branch. Furthermore, the service over the Bank branch is at maximum capacity for the new signalling system.

The Northern line began operating a 24 hour timetable on Friday and Saturday nights on 18 November 2016. The service serves the High Barnet, Edgware, Charing Cross and Morden branches. The service frequency is:

The Northern line is currently being extended from Kennington to Battersea with a short tunnel between the two and one intermediate station. The route was approved during November 2010 by the local authority, Wandsworth Council, with the Mayor of London consenting on December 2010. The route will include new stations to be constructed at Battersea power station and Nine Elms, between Wandsworth Road and Pascal Road. This short extension to the line should open during 2020. Furthermore, a route to connect the line to Clapham Junction station has been kept clear for any further extensions to the line.

The Northern line is the busiest railway on the London Underground network. The railway spans a total of 58km (36 miles) over 6 branches, two in central London and 4 outside central London.

The Northern line currently holds several records on the London Underground network:

• Longest continuous tunnel, East Finchley to Morden via Bank 27.841km (17 miles 528 yards)
• Highest point above ground level, Dollis Brook viaduct on Mill Hill East branch 18m (60ft) above ground level
• Deepest point below ground level, Holly Bush Hill north of Hampstead station 67.4m (221ft) below ground level
• Deepest underground station, Hampstead station platforms 58.8m (192ft) below ground level
• Station with the longest lift shaft, Hampstead station 55.2m (181ft)
• Longest escalators, Angel station, 60m (197ft) in length down a 27.5m (90ft) drop
• Station with the longest lift shaft, Hampstead station 55.2m (181ft)
• Longest escalators, Angel station, 60m (197ft) in length down a 27.5m (90ft) drop
• Most southern station, Morden station at 16km (10 miles) from Central London

The current service frequency on the Northern line: