The Northern line is a London Underground line that runs between North London and South London. It is printed in black on the Tube map. It carries more passengers per year than any other Underground line – around 340 million in 2019 – making it the busiest tube line in London. The Northern line is unique on the Underground network in having two different routes through central London, two southern branches and two northern branches. Despite its name, it does not serve the northernmost stations on the Underground, though it does serve the southernmost station at Morden, the terminus of one of the two southern branches.

The line's northern termini, all in the London Borough of Barnet, are at Edgware and High Barnet; Mill Hill East is the terminus of a single-station branch line off the High Barnet branch. The two main northern branches run south to join at Camden Town where two routes, one via Charing Cross in the West End and the other via Bank in the City, continue and then join at Kennington in Southwark. At Kennington the line again divides into two branches, one to each of the southern termini – at Morden, in the borough of Merton, and at Battersea Power Station in Wandsworth.

For most of its length the Northern line is a deep tube line. The portion between Stockwell and Borough opened in 1890 and is the oldest section of deep-level tube line on the network. Nearly 340 million passenger journeys were recorded in 2019 on the Northern line, making it the busiest on the Underground, although this is distorted due to having 2 branches within Central London, both of which are less busy than the core sections of other lines. It has 18 of the system's 31 stations south of the River Thames. There are 52 stations in total on the line, of which 38 have platforms below ground.

The line has a complicated history. Its longtime structure of two main northern branches, two central branches, and southern unification, reflects its genesis as three separate railways which were combined in the 1920s and 1930s. An extension in the 1920s used a route originally planned by a fourth company. Abandoned plans from the 1920s to extend the line further southwards, and then northwards in the 1930s, would have incorporated parts of the routes of two further companies. From the 1930s to the 1970s, the tracks of a seventh company were also managed as a branch of the Northern line. An extension of the Charing Cross branch from Kennington to Battersea opened on 20 September 2021, giving the line a second southern branch. There are also proposals to split the line into separate lines following the opening of the new link to Battersea.

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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. 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. The railway opened in November 1890 from Stockwell to a now-disused station at King William Street. 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. By 1907, the C&SLR had been further extended at both ends to run from Clapham Common to Euston.

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). It was extended south by one stop to Embankment in 1914 to form an interchange with the Bakerloo and District lines. 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.

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. Construction began in 1922 and this first tunnel opened in 1924. The second connection linking the CCE&HR's Embankment and C&SLR's Kennington stations opened in 1926. 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.

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) which the UERL's subsidiary, the London Electric Railway, had taken over in 1912. 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. 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. Five new stations were built to pavilion-style designs by Stanley Heaps, head of the Underground's Architects Office, stimulating the rapid northward expansion of suburban developments in the following years.

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 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. 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.

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. 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". With Egyptology very much in fashion after the discovery of the tomb of Tutankhamun in 1922, there was also a proposal to call the line the Tootancamden Line as it passed through both Tooting and Camden. It was eventually named the Northern line from 28 August 1937, reflecting the planned addition of the Northern Heights lines.

In June 1935 the LPTB announced the New Works Programme, an ambitious plan to expand the Underground network which included the integration of a complex of existing London and North Eastern Railway (LNER) lines north of Highgate through the Northern Heights. 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 line taken over would be extended beyond Edgware to Brockley Hill, Elstree South and Bushey Heath with a new depot at Aldenham. The extension's route was that planned for the unbuilt Watford and Edgware Railway (W&ER), using rights obtained from the earlier purchase of the W&ER (which had long intended an extension of the EH&LR Edgware route towards Watford). This also provided the potential for further extension in the future; Bushey's town planners reserved space in Bushey village for a future station and Bushey Heath station's design was revised several times to ensure this option would remain available in the future.

The project involved electrification of the surface lines (operated by steam trains at the time), the doubling of the original single-line section between Finchley Central and the proposed junction with the Edgware branch of the Northern line, and the construction of three new linking sections of track: a connection between Northern City Line and Finsbury Park station on the surface; an extension from Archway to the LNER line near East Finchley via new deep-level platforms below Highgate station; and a short diversion from just before the LNER's Edgware station to the Underground's station of the same name.

The peak-hour service pattern was to be 21 trains an hour each way on the High Barnet branch north of Camden Town, 14 of them via the Charing Cross branch and seven via the Bank branch. 14 would have continued on beyond Finchley Central, seven each on the High Barnet and Edgware branches. An additional seven trains an hour would have served the High Barnet branch but continued via Highgate High-Level and Finsbury Park to Moorgate, a slightly shorter route to the city. It does not seem to have been intended to run through trains to the ex-Northern City branch from Edgware via Finchley Central. Seven trains an hour would have served the Alexandra Palace branch, to/from Moorgate via Highgate High-Level. In addition to the 14 through trains described the ex-Northern City branch would have had 14 four-car shuttle trains an hour.

Work began in the late 1930s and was in progress on all fronts by the outbreak of the Second World War. The tunnelling northwards from the original Highgate station (now Archway) had been completed, and the service to the rebuilt surface station at East Finchley started on 3 July 1939 but without the opening of the intermediate (new) Highgate Station at the site of the LNER's station of the same name. Further progress was disrupted by the start of the war; however enough development had been made to complete the electrification of the High Barnet branch onwards from East Finchley (over which tube services started on 14 April 1940) and the new deep-level Highgate station opened on 19 January 1941. The single track LNER line to Edgware was electrified as far as Mill Hill East, including the Dollis Brook Viaduct, opening as a tube service on 18 May 1941 to serve the barracks there thus forming the Northern line as it is today. The new depot at Aldenham had already been built and was used to build Halifax bombers. Work on the other elements of the plan was suspended late in 1939.

Work on the extension from Edgware to Bushey Heath including work on a viaduct and a tunnel started in June 1939, but was stopped after war broke out. 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 thus vanished. Passenger numbers also dropped on the then-BR's Mill Hill and Alexandra Palace branches, so it was useless to electrify them. Available funds were directed towards completing the eastern extension of the Central line instead, and the Northern Heights plan was dropped on 9 February 1954. Aldenham depot was converted into an overhaul facility for buses.

The implemented service from High Barnet branch gave good access both to the West End and the City. This appears to have undermined traffic on the Alexandra Palace branch, still run with steam haulage to Kings Cross via Finsbury Park, as Highgate (low-level) was but a short bus ride away and car traffic was much lighter than it would become later. Consequently, the line from Finsbury Park to Muswell Hill and Alexandra Palace via the surface platforms at Highgate was closed altogether to passenger traffic in 1954. This contrasts with the decision to electrify the Epping-Ongar branch of the Central line, another remnant of the New Works programme, run as a tube-train shuttle from 1957. A local pressure group, the Muswell Hill Metro Group, campaigns to reopen this route as a light-rail service. So far there is no sign of movement on this issue: the route, now the Parkland Walk, is highly valued by walkers and cyclists, and suggestions in the 1990s that it could, in part, become a road were met with fierce opposition. Another pressure group has proposed using the track bed further north, as part of the North and West London Light Railway. The connection between Drayton Park and the surface platforms at Finsbury Park was opened in 1976, when the Northern City Line became part of British Rail.

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.

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. The line gained the nickname "Misery Line" due to its perceived unreliability. 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.

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. The Northern line was originally scheduled to switch to automatic train operation in 2012, using the same SelTrac S40 system as used since 2009 on the Jubilee line and for a number of years on the Docklands Light Railway. 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. The first section of the line (West Finchley to High Barnet) was transferred to the new signalling system on 26 February 2013 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.

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.

Since the mid-autumn of 2016, 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. 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.

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, with an intermediate stop at Nine Elms. Approved by Wandsworth Council in 2010 and TfL in 2014, the construction of the line began in 2015. Tunnelling for the project was completed in 2017, and the extension opened on 20 September 2021. Provision has been made for a future extension to Clapham Junction railway station.

As of September 2021, morning peak southbound services are:

• 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
• 4 tph from High Barnet to Battersea Power Station via Charing Cross
• 6 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, 7 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:

• 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, 10 tph between Kennington and Battersea Power Station 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.

• 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 1907–24 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 1901–22 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 1940
Totteridge & Whetstone				Northern line introduced 14 April 1940
Woodside Park				Northern line introduced 14 April 1940
West Finchley		1 March 1933		Northern line introduced 14 April 1940
Mill Hill East		22 August 1867	Mill Hill branch	Closed 11 September 1939, reopened 18 May 1941
Finchley Central			High Barnet & Mill Hill branches	First Northern line train was 14 April 1940
East Finchley			High Barnet branch	First Northern line train was 3 July 1939
Highgate		19 January 1941		Disused surface station opened 22 August 1867
Archway		22 June 1907		Originally named Highgate
Tufnell Park
Kentish Town				Mainline station opened 13 July 1868. Change for National Rail services.

Station	Image	Opened	Additional information
Edgware		18 August 1924	Terminus
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.
Colindale		18 August 1924	Used as a terminus for some trains travelling north.
Hendon Central		19 November 1923
Brent Cross			Opened as "Brent"; renamed 20 July 1976.
Golders Green		22 June 1907	Originally a terminus; remains a terminus for some trains.
Hampstead			Originally proposed to be named "Heath Street"; this name can still be seen on wall tilings on station platform walls.
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.
Chalk Farm

Station	Image	Opened	Branch	Additional information
Camden Town		22 June 1907	Edgware, High Barnet, Charing Cross and Bank branches	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.

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.
Euston (Charing Cross branch)			Change for southbound Northern line service via Bank from platform 6, Victoria line, Lioness line and National Rail services
Warren Street			Change for Victoria line
Goodge Street			Opened as "Tottenham Court Road"; renamed 3 September 1908
Tottenham Court Road			Change for Central line and Elizabeth line.
Leicester Square			Piccadilly line opened 15 December 1906
Charing Cross			Bakerloo line opened as Trafalgar Square 10 March 1906. Stations combined 1 May 1979. Change for Bakerloo line and National Rail services
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 lines
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 services

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 services
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 Eurostar
Angel		17 November 1901	Has the longest escalator on the entire Underground network
Old Street			Northern line platforms opened February 1904. Connects with National Rail services.
Moorgate		25 February 1900	Metropolitan Railway station opened 23 December 1865. Change for Circle, Hammersmith & City and Metropolitan lines and National Rail services. 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.
London Bridge ( Trains to Gatwick and Luton) ( London Bridge City Pier)			Change for Jubilee line and National Rail services
Borough		18 December 1890
Elephant & Castle			Change for Bakerloo line and National Rail services

Station	Image	Opened	Branch	Additional information
Kennington		18 December 1890	Charing Cross, Bank, Battersea and Morden branches	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.

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

Station	Image	Opened	Additional information
Oval		18 December 1890
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.
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.
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.
Clapham South		13 September 1926	One of the eight stations that have a deep level air-raid shelter.
Balham		6 December 1926	Change for National Rail services
Tooting Bec		13 September 1926	Opened as "Trinity Road"; renamed 1 October 1950
Tooting Broadway			Used as a terminus for some trains heading south
Colliers Wood
South Wimbledon			Opened as "South Wimbledon (Merton)". The suffix gradually fell out of use, but still can be seen on some platform signage.
Morden			Terminus

• 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. 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. 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, 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. There are also time-based restrictions for the sections where bicycles are allowed.

The tunnel from Morden to East Finchley via Bank, 17 miles 528 yards (27.841 km), 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, adjacent to Golders Green tube station, while the second, at Morden, 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 separate routes. 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. Completely segregating the routes could allow 36 trains an hour on all parts of the line, increasing capacity by around 25%.

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. The extension to Battersea would allow the Charing Cross branch to terminate at Battersea Power Station.

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. 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. 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. In 2018, plans to upgrade and rebuild Camden Town station were placed indefinitely on hold, due to TfL's financial situation. As of 2022, there is currently no plan to split the line.

In October 2003, a train derailed at Camden Town. 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. 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.

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.

• In his debut novel Ghostwritten, David Mitchell characterises the Northern line as "the psycho of the family".
• The Bloc Party song "Waiting For the 7.18" references the Northern line as "the loudest".
• 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.
• The Nick Drake song "Parasite" references the Northern Line.
• 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."
• The 2021 Maisie Peters song "Elvis Song" begins, "Cold bench on a platform/Last train on the Northern Line."

• 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

• Above-ground route of line from Morden to Edgware, constructed from Google StreetView
• Architectural history of London Underground during 1920-30s from the Royal Institute of British Architects
  • Old Street, Angel and Stockwell stations
  • South Clapham, Tooting Bec and South Wimbledon