Northern line

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The core of the Northern line evolved from two railway companies: the City & South London Railway (C&SLR) and the Charing Cross, Euston & Hampstead Railway (CCE&HR).

The C&SLR was London's first electric hauled deep-level tube railway. It was built under the supervision of James Henry Greathead, who had been responsible (with Peter W. Barlow) for the Tower Subway.^[5] It was the first of the Underground's lines to be constructed by boring deep below the surface and the first to be operated by electric traction.^[6] The railway opened in November 1890 from Stockwell to a now-disused station at King William Street.^[7] This was inconveniently placed and unable to cope with the company's traffic so in 1900 a new route to Moorgate via Bank was opened.^[8] By 1907, the C&SLR had been further extended at both ends to run from Clapham Common to Euston.^[9]

The CCE&HR (commonly known as the "Hampstead Tube") was opened in 1907 and ran from Charing Cross (known for many years as Strand) via Euston and a junction at Camden Town to Golders Green and Highgate (now known as Archway).^[10][11] It was extended south by one stop to Embankment in 1914 to form an interchange with the Bakerloo and District lines.^[11][12] In 1913, the Underground Electric Railways Company of London (UERL), owner of the CCE&HR, took over the C&SLR although they remained separate companies.^[13]

During the early 1920s a series of works were carried out which involved connecting the C&SLR and CCE&HR tunnels to enable an integrated service to be operated. The first of these new tunnels, between the C&SLR's Euston station and the CCE&HR's station at Camden Town, had originally been planned in 1912 but was delayed by the First World War.^[14][15] Construction began in 1922 and this first tunnel opened in 1924.^[11][15] The second connection linking the CCE&HR's Embankment and C&SLR's Kennington stations opened in 1926.^[11][15] It provided a new intermediate station at Waterloo to connect to the main line station there and the Bakerloo line. The smaller diameter tunnels of the C&SLR were also enlarged to match the standard diameter of the CCE&HR and other deep tube lines.^[16]

In conjunction with the works to integrate the two lines, two major extensions were undertaken: northwards to Edgware in Middlesex (now in the London Borough of Barnet) and southwards to Morden in Surrey (then in the Merton and Morden Urban District, but now in the London Borough of Merton).

The Edgware extension used plans dating back to 1901 for the Edgware and Hampstead Railway (E&HR)^[17] which the UERL's subsidiary, the London Electric Railway, had taken over in 1912.^[18] It extended the CCE&HR line from its terminus at Golders Green to Edgware in two stages: to Hendon Central in 1923 and to Edgware in 1924.^[11][19] The line crossed open countryside and ran mostly on viaduct from Golders Green to Brent and then on the surface, apart from a short tunnel north of Hendon Central.^[19] Five new stations were built to pavilion-style designs by Stanley Heaps, the UERL's architect, stimulating the rapid northward expansion of suburban developments in the following years.^[20]

The engineering of the Morden extension of the C&SLR from Clapham Common to Morden was more demanding; it runs in tunnels to a point just north of Morden station which was constructed in a cutting. The line then goes under the wide station forecourt and public road outside the station to the depot. The extension was initially planned to continue to Sutton^[21] over part of the route for the unbuilt Wimbledon and Sutton Railway (in which the UERL held a stake) but agreements were made with Southern Railway to end the extension at Morden. Southern Railway built the surface line from Wimbledon to Sutton in the 1930s via South Merton and St. Helier.^[b] The tube extension itself opened in 1926 with seven new stations all designed by Charles Holden in a modern style. Originally Stanley Heaps was to design the stations, but after seeing the designs Frank Pick (Assistant Joint Manager of the UERL) decided that Holden should take over the project.^[22]

With the exception of Morden and Clapham South, where more land was available, the new stations were built on confined corner sites at main road junctions in areas that had been already developed. Holden made good use of this limited space and designed striking buildings. The street-level structures are of white Portland stone with tall double-height ticket halls, with the London Underground roundel made up in coloured glass panels in large glazed screens. The stone columns framing the glass screens are surmounted by a capital formed as a three-dimensional version of the roundel. The large expanses of glass above the entrances ensure that the ticket halls are bright and lit from within at night, creating a welcoming feel.^[23] The first and last new stations on the extension, Clapham South and Morden, include a parade of shops and were designed with structures capable of being built above (like many of the earlier central London stations). Clapham South was extended upwards soon after its construction with a block of apartments; Morden was extended upwards in the 1960s with a block of offices. All the stations on the extension, except Morden itself, are Grade II listed buildings.

After the UERL and the Metropolitan Railway became part of the new unified London Passenger Transport Board in 1933, the MR's subsidiary the Great Northern & City Railway became part of the Underground as the Northern City Line. It ran underground from Moorgate to Finsbury Park. In preparation for the Northern Heights Plan it was operated as part of the Northern line, although it was never connected to the rest of the line.

The resulting line became known as the Morden–Edgware line, although a number of alternative names were also mooted in the fashion of the contraction of Baker Street & Waterloo Railway to "Bakerloo", such as "Edgmor", "Mordenware", "Medgeway" and "Edgmorden".^[24] It was eventually named the Northern line from 28 August 1937,^[25] reflecting the planned addition of the Northern Heights lines.^[26]

Following the formation of the LPTB, in June 1935 the organisation proposed the New Works Programme, an ambitious plan to expand the Underground network in response to London's growing suburban population which included the integration of existing London and North Eastern Railway (LNER) suburban lines north of Highgate. These lines, built in the 1860s and 1870s by the Edgware, Highgate and London Railway (EH&LR) and its successors, ran from Finsbury Park to Edgware via Highgate, with branches to Alexandra Palace and High Barnet.

The Morden–Edgware line's project involved electrification of the steam-operated surface lines and the double-tracking of the single-line section between Finchley (Church End) and Edgware. The Northern Heights also called for the construction of three new linking sections of track, mainly between the Northern City line and new surface level platforms at Finsbury Park; a deep-level tunnel from Archway to East Finchley; and a diversion of the Mill Hill branch to LPTB's Edgware station.^[27]

In addition, the Edgware branch would also be extended beyond the terminus to a site at Bushey Heath, the LPTB having retained planning rights of the unbuilt Watford and Edgware Railway (W&ER), which had long intended an extension of the EH&LR's Edgware branch towards Watford. A new depot at Aldenham would also be constructed to facilitate the housing of additional trains for the Northern line.^[28]

Work began on the initial stages of the extensions in 1936, as did that on Bushey Heath following its authorisation in 1937.^[27] The tunnelling northwards from Archway were the first to be completed and an initial service to the rebuilt East Finchley station commenced on 3 July 1939, though prior to the opening of the new deep-level platforms at Highgate, which tube trains called from 19 January 1941.

Further progress was disrupted by the start of the Second World War in September 1939; however enough development had been made to complete the electrification of the High Barnet branch, over which tube services started on 14 April 1940. The single track LNER line to Edgware was electrified as far as Mill Hill East, reopening as a tube service on 18 May 1941 to serve the nearby Inglis Barracks. The new depot at Aldenham had also been built and was used to build Halifax bombers.^[28]

Other work on the extension that were eventually halted during the Second World War included the construction of a viaduct and a tunnel at Brockley Hill and Elstree South which started in June 1939, the laying of a second line as far as Mill Hill (The Hale) alongside the construction of a second platform at that station.^[29]

After the war, the area beyond Edgware was made part of the Metropolitan Green Belt, largely preventing the anticipated residential development in the area and the potential demand for services from Bushey Heath vanished.^[28] A compromise was offered to make Brockley Hill the line's terminus and retaining a link to Aldenham Depot,^[27] but the Bushey Heath extension was cancelled in 1950.^[30]

The introduction of electric services to High Barnet and Mill Hill East undermined passenger numbers on the remaining LNER-operated lines. Consequently, the line from Finsbury Park to Muswell Hill and Alexandra Palace via the surface platforms at Highgate was closed altogether to passenger traffic on 3 July 1954, with the last of the Northern Heights plans also being dropped that year.^[27] Available funds were directed towards completing the eastern extension of the Central line instead. Aldenham Depot was later converted into an overhaul facility for buses.

The connection between Drayton Park and the surface platforms at Finsbury Park, which gave access onto the Finsbury Park–Highgate line, was maintained to allow rolling stock transfers between the Northern City and Northern line until 1970.^[27] Passenger services commenced on the Finsbury Park link in 1976, when the Northern City line transferred to British Rail ownership.

In the 1980s, a southern extension of the line to Peckham was proposed, as part of a review of potential extensions of Underground lines. The proposal was not proceeded with.^[31]

By the early 1990s, the line had deteriorated due to years of under-investment and the use of old rolling stock, most of which dated back to the early 1960s.^[32] The line gained the nickname "Misery Line" due to its perceived unreliability.^[33][34] In 1995, a comprehensive refurbishment of the line began – including track replacement, power upgrades, station modernisation (such as Mornington Crescent) and the replacement of older rolling stock with new 1995 Stock thanks to a public–private partnership deal with Alstom.^[35][36]

Throughout the 2000s, no plans were considered for extending the Northern line, as the PPP to upgrade the Underground did not include provision for line extensions within the PPP contracts.^[37][38] The Northern line was originally scheduled to switch to automatic train operation in 2012, using the same SelTrac S40 system^[39] as used since 2009 on the Jubilee line and for a number of years on the Docklands Light Railway.^[40] Originally the work was to follow on from the Jubilee line so as to benefit from the experience of installing it there, but that project was not completed until spring 2011. Work on the Northern line was contracted to be completed before the 2012 Olympics. It was then undertaken in-house, and TfL predicted the upgrade would be complete by the end of 2014.^[41] The first section of the line (West Finchley to High Barnet) was transferred to the new signalling system on 26 February 2013^[42] and the line became fully automated on 1 June 2014, with the Chalk Farm to Edgware via Golders Green section being the last part of the line to switch to ATO.^[43][44]

In January 2018, Transport for London announced that it would double the period during which it runs peak evening services in the central London section to tackle overcrowding. There would now be 24 trains per hour on both central London branches and the northern branches, as well as 30 trains per hour on the Kennington to Morden section between 5 pm and 7 pm.^[4]

Since the mid-autumn of 2016,^[45] a 24-hour "Night Tube" service has run on Friday and Saturday nights from Edgware and High Barnet to Morden via the Charing Cross branch; service is suspended on the Bank branch during these times.^[46] Trains run every eight minutes between Morden and Camden Town and every 15/16 minutes between Camden Town and Edgware/High Barnet. Labour disputes delayed the planned start date of September 2015.^[47]

In September 2021, the Northern line was extended to serve the redevelopment of Battersea Power Station. Partially funded by private developers, the £1.2bn project extended the Charing Cross branch of the line for 3.2 km (2.0 miles) from Kennington to Battersea Power Station,^[48] with an intermediate stop at Nine Elms.^[49][50] Approved by Wandsworth Council in 2010^[51] and TfL in 2014,^[49] the construction of the line began in 2015. Tunnelling for the project was completed in 2017,^[48] and the extension opened on 20 September 2021.^[52][53] Provision has been made for a future extension to Clapham Junction railway station.^[54]

As of September 2021, morning peak southbound services are:^[55]

• 4 tph from Edgware to Battersea Power Station via Charing Cross
• 2 tph from Edgware to Morden via Charing Cross
• 12 tph from Edgware to Morden via Bank
• 10 tph from High Barnet to Battersea Power Station via Charing Cross
• 2 tph from High Barnet to Morden via Charing Cross
• 8 tph from High Barnet to Morden via Bank
• 2 tph from Mill Hill East to Battersea Power Station via Charing Cross
• 2 tph from Mill Hill East to Morden via Bank

This service pattern provides 20 tph between Finchley Central and High Barnet, 4 tph between Finchley Central and Mill Hill East, 16 tph between Kennington and Battersea Power Station and 22 tph everywhere else on the line except between Kennington and Morden, between Camden Town and Finchley Central and on the Edgware branch where there will be 24 tph.

As of November 2022, off-peak services are the similar to peak services, minus the four hourly trains that run from Morden to the northern branches via Charing Cross:^[55]

• 10 tph from Edgware to Battersea Power Station via Charing Cross
• 10 tph from Edgware to Morden via Bank
• 8 tph from High Barnet to Battersea Power Station via Charing Cross
• 8 tph from High Barnet to Morden via Bank
• 2 tph from Mill Hill East to Battersea Power Station via Charing Cross
• 2 tph from Mill Hill East to Morden via Bank

This service pattern provides 16 tph between Finchley Central and High Barnet, 4 tph between Finchley Central and Mill Hill East, and 20 tph everywhere else on the line.

Since 2016, the Northern line has operated Night Tube services on Friday and Saturday nights between the Edgware and High Barnet termini and Morden, via the Charing Cross branch only. Trains run every 15 minutes on each of the northern branches, combining to give eight trains per hour between Camden Town and Morden. There is no Night Tube service on the Mill Hill East, Bank, or Battersea branches.^[45]

• 4 tph from High Barnet to Morden via Charing Cross
• 4 tph from Edgware to Morden via Charing Cross

v t e Northern line
Legend
Bushey Heath Aldenham Works Elstree South cancelled Bushey Heath extension Brockley Hill Edgware Edgware (LNER) 1867–1939 Edgware, Highgate & London Railway Edgware depot proposed but unbuilt connection Burnt Oak Mill Hill (The Hale) Mill Hill East High Barnet Colindale Dollis Brook Viaduct Totteridge & Whetstone Woodside Park Burroughs tunnel West Finchley Hendon Central Finchley Central East Finchley Alexandra Palace Brent Cross Muswell Hill Cranley Gardens Muswell Hill branch 1873–1954 Golders Green Highgate depot Golders Green depot Highgate north tunnel Highgate not completed North End Archway Tufnell Park Hampstead Kentish Town Belsize Park Edgware, Highgate & London Railway 1867–1954 South Kentish Town 1907–1924 Chalk Farm Crouch End Stroud Green Finsbury Park Camden Town Northern City Line Drayton Park Mornington Crescent Highbury and Islington Euston Essex Road Warren Street King's Cross St Pancras Goodge Street Angel City Road 1901–1922 Tottenham Court Road Old Street Moorgate Leicester Square Lothbury authorised but not built Bank (Monument ) Charing Cross King William Street 1890–1900 River Thames Embankment London Bridge 1914–25 Charing Cross reversing loop Waterloo Borough Elephant & Castle Kennington reversing loop Oval Stockwell depot closed 1915 Nine Elms Stockwell Battersea Power Station Clapham North reserved route Clapham Common Clapham Junction Clapham South Balham Tooting Bec Tooting Broadway Colliers Wood South Wimbledon Morden Wimbledon & Sutton Rly to Wimbledon Morden depot Morden South proposed but unbuilt connection South Morden authorised but not built Sutton Common Cheam authorised but not built Sutton

Station	Image	Opened	Branch	Additional information
High Barnet		1 April 1872	High Barnet branch	Terminus. Northern line introduced 14 April 1940map 1
Totteridge & Whetstone				Northern line introduced 14 April 1940 map 2
Woodside Park				Northern line introduced 14 April 1940map 3
West Finchley		1 March 1933		Northern line introduced 14 April 1940map 4
Mill Hill East		22 August 1867	Mill Hill branch	Closed 11 September 1939, reopened 18 May 1941map 5
Finchley Central			High Barnet & Mill Hill branches	First Northern line train was 14 April 1940map 6
East Finchley			High Barnet branch	First Northern line train was 3 July 1939map 7
Highgate		19 January 1941		Disused surface station opened 22 August 1867map 8
Archway		22 June 1907		Originally named Highgatemap 9
Tufnell Park				map 10
Kentish Town				Mainline station opened 13 July 1868. Change for National Rail services.map 11

Station	Image	Opened	Additional information
Edgware		18 August 1924	Terminusmap 12
Burnt Oak		27 October 1924	Opened with its current name, then renamed as "Burnt Oak (Watling)" approximately four years after its opening; was reverted to its original name in 1950.map 13
Colindale		18 August 1924	Used as a terminus for some trains travelling north.map 14
Hendon Central		19 November 1923	map 15
Brent Cross			Opened as "Brent"; renamed 20 July 1976.map 16
Golders Green		22 June 1907	Originally a terminus; remains a terminus for some trains.map 17
Hampstead			Originally proposed to be named "Heath Street"; this name can still be seen on wall tilings on station platform walls.map 18
Belsize Park			One of eight London Underground stations that have deep-level air-raid shelters underneath them. The shelter was constructed in the Second World War to provide safe accommodation for service personnel.map 19
Chalk Farm			map 20

Station	Image	Opened	Branch	Additional information
Camden Town		22 June 1907	Edgware, High Barnet, Charing Cross and Bank branches[c]	The junctions connecting the two northern branches of the Northern line to the two central branches are just south of Camden Town station. The station has a pair of platforms on each of the two northern branches, and southbound trains can depart toward either Charing Cross or Bank from either of the two southbound platforms without crossing over.map 21

Station	Image	Opened	Additional information
Mornington Crescent		22 June 1907	Was planned to be named "Seymour Street", but this was changed. It was closed on 23 October 1992 to replace the lifts and was reopened on 27 April 1998.map 22
Euston (Charing Cross branch)			Change for southbound Northern line service via Bank from platform 6, Victoria line, Lioness line and National Rail servicesmap 23
Warren Street			Change for Victoria linemap 24
Goodge Street			Opened as "Tottenham Court Road"; renamed 3 September 1908map 25
Tottenham Court Road			Change for Central line and Elizabeth line.
Leicester Square			Piccadilly line opened 15 December 1906 map 27
Charing Cross			Bakerloo line opened as Trafalgar Square 10 March 1906. Stations combined 1 May 1979. Change for Bakerloo line and National Rail servicesmap 28
Embankment ( Embankment Pier)		6 April 1914	District Railway opened 30 May 1870. Northern line extension from Charing Cross opened 6 April 1914. Extension from Kennington opened 13 September 1926. Change for Bakerloo, Circle and District linesmap 29
Waterloo ( Waterloo Pier, Festival Pier)		13 September 1926	Waterloo and City line opened 8 August 1898. Extension from Kennington opened 13 September 1926. Change for Bakerloo, Jubilee and Waterloo & City lines and National Rail servicesmap 30

Station	Image	Opened	Additional information
Euston (Bank branch)		12 May 1907	Change for southbound Northern line service via Charing Cross from platform 2, Victoria line, Lioness line and National Rail servicesmap 23
King's Cross St Pancras ( Trains Gatwick and Luton)			Metropolitan Railway station opened 10 January 1863. Change for Circle, Hammersmith & City, Metropolitan, Piccadilly and Victoria lines, National Rail services and Eurostarmap 31
Angel		17 November 1901	Has the longest escalator on the entire Underground networkmap 32
Old Street			Northern line platforms opened February 1904. Connects with National Rail services.map 33
Moorgate		25 February 1900	Metropolitan Railway station opened 23 December 1865. Change for Circle, Hammersmith & City and Metropolitan lines and National Rail services.map 34 Has an interchange with the Elizabeth line via Liverpool Street station.
Bank			Linked with Monument by escalator 18 September 1933, change for Central, Circle, District and Waterloo & City lines and Docklands Light Railway.map 35
London Bridge ( Trains to Gatwick and Luton) ( London Bridge City Pier)			Change for Jubilee line and National Rail servicesmap 36
Borough		18 December 1890	map 37
Elephant & Castle			Change for Bakerloo line and National Rail servicesmap 38

Station	Image	Opened	Branch	Additional information
Kennington		18 December 1890	Charing Cross, Bank, Battersea and Morden branches[d]	The station has four platforms arranged in two pairs: one pair for northbound services to each central branch of the Northern line, the other pair for southbound services from each central branch. The junctions connecting the central branches to the southern branches are just south of Kennington station. Southbound trains from the Charing Cross branch can terminate at this station, which has a reversing loop, or join either southern branch; southbound trains from the Bank branch can proceed onto the Morden branch but not the Battersea branch.map 39

Station	Image	Opened	Additional information
Nine Elms		20 September 2021
Battersea Power Station			Terminus

Station	Image	Opened	Additional information
Oval		18 December 1890	map 40
Stockwell			Change for Victoria line. Original terminus until 1900, when the line was extended to Clapham Common. The station platforms were resited south of the original island platform. Formerly a depot existed here; it was branched off from the current southbound track. It is one of the eight stations that have a deep level air-raid shelter. map 41
Clapham North		3 June 1900	One of the two remaining stations to have an island platform underground. It is also one of the eight stations that have a deep level air-raid shelter.map 42
Clapham Common			Terminus from 1900 to 1926. It is also one of the two remaining stations to have an island platform underground. It is also one of the eight stations that have a deep level air-raid shelter.map 43
Clapham South		13 September 1926	One of the eight stations that have a deep level air-raid shelter.map 44
Balham		6 December 1926	Change for National Rail servicesmap 45
Tooting Bec		13 September 1926	Opened as "Trinity Road"; renamed 1 October 1950map 46
Tooting Broadway			Used as a terminus for some trains heading southmap 47
Colliers Wood			map 48
South Wimbledon			Opened as "South Wimbledon (Merton)". The suffix gradually fell out of use, but still can be seen on some platform signage.map 49
Morden			Terminusmap 50

• King William Street (closed 1900; replaced by Bank)
• City Road (closed 1922)
• South Kentish Town (closed 1924)
• North End (also known as "Bull & Bush". Never opened; work stopped 1906)

• Stockwell – new platforms resited immediately to the south of its predecessor with the 1922–1924 upgrade of the line.
• London Bridge – the northbound tunnel and platform converted into a concourse, and a new northbound tunnel and platform built in the late 1990s to increase the platform and circulation areas in preparation for the opening of the Jubilee line.

• Highgate – High-level only
• Stroud Green
• Crouch End
• Cranley Gardens
• Muswell Hill
• Alexandra Palace
• Mill Hill (The Hale)

• Brockley Hill
• Elstree South
• Bushey Heath

When the line opened, it was served by 1906 Stock. This was replaced by Standard stock which was in turn replaced by 1938 stock as part of the New Works Programme, later supplemented with identical 1949 Stock. When the Piccadilly line was extended to Heathrow Airport in the 1970s, its 1959 Stock and 1956 Stock (prototypes of the 1959 Stock) trains were transferred to the Northern line. As there were not enough 1956 and 1959 Stock trains to replace the Northern line's 1938 Stock fleet, they were supplemented with newly built 1972 Mark 1 Stock trains, which all served the line at the same time. 1972 Mark 2 stock trains also ran on this line until going to the Jubilee line; they were then moved to the Bakerloo line, where they remain in service. By 1986, increasing unreliability of 1959 stock trains meant there were regularly too few trains to run a full peak service. Five 1938 stock trains, newly retired from the Bakerloo Line, were overhauled and returned to the Northern Line to serve another two years until further 1972 stock could be transferred from the Jubilee Line, which was moving to 1983 stock. The few 1956 Stock trains were briefly replaced by 1962 Stock transferred from the Central line in 1995, before the entire Northern line fleet was replaced with 1995 Stock between 1998 and 2000.

Today, all Northern line trains consist of 1995 Stock in the Underground livery of red, white and blue. In common with the other deep-level lines, the trains are the smaller of the two loading gauges used on the system. 1995 stock has automated announcements and quick-close doors.^{[citation needed]} If the proposed split of the line takes place (initial estimates of 2018 having been abandoned to focus on completion of the Battersea and Nine Elms extension work), 19 new trains will be added to the existing fleet of 106 trains,^[56] though additional trains beyond the extra 19 trains may be required to provide a full service for the new Battersea extension.

Although two other London Underground lines operate fully underground, the Northern line is unusual in that it is a deep-level tube line that serves the outer suburbs of South London yet there is only one station above ground (Morden tube station) while the rest of this part of the line is deep below ground. The short section to Morden depot is also above ground. This is partly because its southern extension into the outer suburbs was not done by taking over an existing surface line as was generally the case with routes such as the Central, Jubilee and Piccadilly lines. Apart from the core central underground tunnels, part of the section between Hendon and Colindale is also underground. As bicycles are not allowed in tunnel sections (even if no station is in that section) as they would hinder evacuation, they are limited to High Barnet – East Finchley, the Mill Hill East branch, Edgware – Colindale and Hendon Central – Golders Green.^[57] There are also time-based restrictions for the sections where bicycles are allowed.^[57]

The tunnel from Morden to East Finchley via Bank, 17 miles 528 yards (27.841 km),^[1] was for a time the longest rail tunnel in the world. Other tunnels, including the Channel Tunnel that links the UK and France, are now longer.

The Northern line is serviced by four depots. The main one is at Golders Green^{map 51}, adjacent to Golders Green tube station, while the second, at Morden,^{map 52} is south of Morden tube station and is the larger of the two. The other two are at Edgware and Highgate. The Highgate depot is on the former LNER branch to Alexandra Palace. There was originally a depot at Stockwell, but this closed in 1915. There are sidings at High Barnet for stabling trains overnight.

Since the 2000s, TfL has aspired to split the Northern line into two routes.^[58][59] Running trains between all combinations of branches and the two central sections, as at present, means only 24 trains an hour can run through each of the central sections at peak times, because merging trains have to wait for each other at the junctions at Camden Town and Kennington.^[60] Completely segregating the routes could allow 36 trains an hour on all parts of the line, increasing capacity by around 25%.^[58][60]

TfL has already separated the Charing Cross and Bank branches during off-peak periods; however, four trains per hour still run to and from Morden via Charing Cross in the peak; the northern branches to Edgware and High Barnet cannot be separated until Camden Town station is upgraded to cope with the numbers of passengers changing trains.^[61] The extension to Battersea would allow the Charing Cross branch to terminate at Battersea Power Station.^[62][63]

The proposed split of the Northern line would require Camden Town station to be expanded and upgraded, as the station is already severely overcrowded at weekend peak times, and a split would increase the number of passengers wishing to change trains at the station.^[63][64][61] In 2005, London Underground failed to secure planning permission for a comprehensive upgrade plan for Camden Town tube station that would have involved demolition of the existing station entrance and several other surface-level buildings, all within a conservation area.^[65][66] New redevelopment plans were first announced in 2013 by TfL, which proposed avoiding the existing station entrance and the conservation area by building a second entrance and interchange tunnels to the north, mostly on the site of a subsequently vacated infant school.^[64] In 2018, plans to upgrade and rebuild Camden Town station were placed indefinitely on hold, due to TfL's financial situation.^[67] As of 2024^[update], TfL said they still "aspire" to split the line. A partial separation was proposed in which all trains from Morden would operate via Bank, while those starting at Kennington (or Battersea) would serve the Charing Cross branch. The High Barnet and Edgware branches would remain served by trains from both routes.^[68]

In October 2003, a train derailed at Camden Town.^[69] Although no one was hurt, points, signals and carriages were damaged. Concern was raised about the safety of the Tube, given the derailment at Chancery Lane earlier in 2003.^[70] A joint report by the Underground and its maintenance contractor Tube Lines concluded that poor track geometry was the main cause, and therefore extra friction arising out of striations (scratches) on a newly installed set of points had allowed the leading wheel of the last carriage to climb the rail and derail. The track geometry at the derailment site is a very tight bend and tight tunnel bore, which precludes the normal solution for this sort of geometry of canting the track by raising the height of one rail relative to the other.^[71]

In August 2010, a defective rail grinding train caused disruption on the Charing Cross branch, after it travelled four miles in 13 minutes without a driver. The train was being towed to the depot after becoming faulty. At Archway station, the defective train became detached and ran driverless until coming to a stop at an incline near Warren Street station. This caused morning rush-hour services to be suspended on this branch. All passenger trains were diverted via the Bank branch, with several not stopping at stations until they were safely on the Bank branch.^[72][73]

• In his debut novel Ghostwritten, David Mitchell characterises the Northern line as "the psycho of the family".^[74]
• The Bloc Party song "Waiting For the 7.18" references the Northern line as "the loudest".^[75]
• As part of a series of twelve books tied to the twelve lines of the London Underground, A Northern Line Minute focuses on the Northern line.^[76]
• The New Vaudeville Band's 1967 song "Finchley Central" ("On Tour" in the US) mentions several stations on the line.^[77]
• The Nick Drake song "Parasite" references the Northern Line.^[78]
• The 1982 Robyn Hitchcock song "Fifty Two Stations" begins, "There's fifty-two stations on the Northern Line/None of them is yours, one of them is mine."^[79]
• The 2021 Maisie Peters song "Elvis Song" begins, "Cold bench on a platform/Last train on the Northern Line."^[80]

• T. P. Figgis, architect of the City and South London Railway's original stations
• Leslie Green, architect of the Charing Cross, Euston and Hampstead Railway's early stations
• List of crossings of the River Thames
• London deep-level shelters, most of which are under Northern line stations
• Tunnels underneath the River Thames