Northern line
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 linearound 340million in 2019making it the busiest tube line in London. The Northern line is unique on the network in having two routes through Central London, two northern branches and two southern 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 and ; is the terminus of a single-station branch line off the High Barnet branch. The two main northern branches run south to join at where two routes, one via in the West End and the other via in the City, continue and then join at in Southwark. At Kennington the line again divides into two branches, one to each of the southern termini – at, in the borough of Merton, and at in Wandsworth.
For most of its length the Northern line is a deep tube line. The portion between and 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 two branches within Central London, both of which are less busy than the core sections of other lines. Despite the line's name, 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's structure of two northern branches, two central branches, and two southern branches reflects its complicated history. The core of the line, including the two central branches and the beginnings of the two northern branches, was constructed by two companies, the City and South London Railway and the Charing Cross, Euston and Hampstead Railway, in the 1890s and 1900s. The companies came under the same ownership in 1913, and were physically connected and operationally merged in the 1920s, while at the same time extensions to Edgware and Morden were completed. In the 1930s and 1940s the Northern line took over and electrified the London and North Eastern Railway branches to Mill Hill East and High Barnet. This was the final extension of the line for eight decades, though between the 1930s and 1970s the Northern City Line was branded and operated as part of the Northern line despite being disconnected from the rest of the line. The most recent extension, a second southern branch from Kennington to Battersea, opened on 20 September 2021. There are proposals to split the line into two lines.
History
Formation
The core of the Northern line evolved from two railway companies: the City & South London Railway and the Charing Cross, Euston & Hampstead Railway.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 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 Strand via Euston to Camden Town with branches splitting to Golders Green and Highgate. It was extended south by one stop to Charing Cross in 1914 to form an interchange with the Bakerloo and District lines. In 1913, the Underground Electric Railways Company of London, owner of the CCE&HR, took over the C&SLR although they remained separate companies for some time.
Integration
During the early 1920s a series of works were carried out to connect 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 and CCE&HR's Camden Town stations 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. 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 and southwards to Morden in Surrey.The Edgware extension used plans dating back to 1901 for the Edgware and Hampstead Railway 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 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, 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, running in tunnel to Morden station which was then constructed in a cutting and the line continued a bit beyond to the depot. The extension was initially planned to continue to Sutton over part of the route of the unbuilt Wimbledon and Sutton Railway but agreements were made with Southern Railway to end the extension at Morden, the SR building a 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.
Owing to the complicated nature of the resulting line, it became known as the Morden–Edgware line, although a number of alternative portmanteau names were mooted in the fashion of the Baker Street and Waterloo Railway becoming the Bakerloo line, such as "Edgmor", "Mordenware", "Medgeway" and "Edgmorden" lines.
After the UERL and the Metropolitan Railway became unified under the London Passenger Transport Board in 1933, the MR's Great Northern & City Railway subsidiary, which ran mostly underground from Moorgate to Finsbury Park, transferred management to the Morden–Edgware line, branding itself as the Northern City line.
Northern Heights plan
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 and relieve congestion on the existing steam-operated suburban lines. In the case of the Morden–Edgware, these were the London and North Eastern Railway suburban lines north of Highgate, built in the 1860s and 1870s by the Edgware, Highgate and London Railway and its successors, running from Finsbury Park to Edgware via Highgate, with branches to Alexandra Palace and High Barnet.The Morden–Edgware line's project, known as the Northern Heights plan owing to the high ground in north London, involved fourth-rail electrification of the surface lines and the double-tracking of the single-line section between Finchley and Edgware. The Northern Heights plan also called for the construction of three new linking sections of track: 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 the LPTB's Edgware station.
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, which had long intended an extension of the EH&LR's Edgware branch towards Watford. A new depot at Aldenham, just before the Bushey Heath terminus, would also be constructed to facilitate the housing of additional trains. As a result of the project's name, the Morden–Edgware line was renamed as the Northern line.
Work began on the initial stages of the extensions in 1936, as did that on Bushey Heath following its authorisation in 1937, with completion projected by 1941. The tunnelling northwards from Archway was the first element to be completed and an initial service to the rebuilt East Finchley station commenced on 3 July 1939, though trains skipped the deep-level platforms at Highgate until its fitout was completed by 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, and the single-track LNER line to Edgware being electrified as far as Mill Hill East, reopening as a tube service on 18 May 1941 to serve the nearby Inglis Barracks. The partially-complete depot at Aldenham was converted into an aircraft factory, constructing Handley Page Halifax bombers as part of the war effort. Other work on the extension that were eventually halted during the Second World War included the construction of a viaduct at Brockley Hill and a tunnel near Elstree South which started in June 1939, the laying of a second line as far as Mill Hill and the construction of its second platform.
After the war, much of the area beyond Edgware was made part of the Metropolitan Green Belt that largely prevented the anticipated residential development, thus the potential demand for services from Bushey Heath vanished. A compromise was offered to make Brockley Hill the line's terminus and retain a link to Aldenham Depot, but the Bushey Heath extension was cancelled in 1950. Although efforts were made to complete Aldenham Depot as an Underground facility, from December 1947 it was modified for use as a heavy repair works of bus bodies, supposedly temporarily until Aldenham was required for railway purposes. Following the extension's cancellation, the depot was converted into an overhaul facility for buses, serving this purpose until 1986.
The introduction of electric services to High Barnet and Mill Hill East undermined passenger numbers on the remaining LNER-operated lines. Consequently, passenger services to Mill Hill and Edgware, having been suspended in September 1939 to allow works to be completed, never resumed and the Alexandra Palace branch line via the surface platforms at Highgate ceased to passenger traffic on 3 July 1954, with the last of the Northern Heights plans also being dropped that year. Available funds were directed towards completing the eastern extension of the Central line instead. Freight-hauled mainline trains continued to run alongside Northern line services until 1964. Tickets were still being sold to and from Mill Hill until the late 1960s, with passengers being directed onto the 240 bus to connect with the Underground.
The connection between Drayton Park and the surface platforms at Finsbury Park, which gave access onto the Finsbury Park–Highgate line, was retained for rolling stock transfers between the Northern City and Northern line until 1970, the incomplete electrification on this section necessitating stock being hauled by battery-electric locomotives. Passenger services commenced on the Finsbury Park link in 1976, when the Northern City line transferred to British Rail ownership.