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.

[nb 1] The portion between Stockwell and Borough opened in 1890 and is the oldest section of deep-level tube line on the network.

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.

It was built under the supervision of James Henry Greathead, who had been responsible (with Peter W. Barlow) for the Tower Subway.

[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).

[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.

[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, head of the Underground's Architects Office, stimulating the rapid northward expansion of suburban developments in the following years.

Southern Railway built the surface line from Wimbledon to Sutton in the 1930s via South Merton and St.

[nb 3] The tube extension itself opened in 1926 with seven new stations all designed by Charles Holden in a modern style.

[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.

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 line taken over would be extended beyond Edgware to Brockley Hill, Elstree South and Bushey Heath with a new depot at Aldenham.

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.

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.

The implemented service from High Barnet branch gave good access both to the West End and the City.

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.

[citation needed] 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.

[citation needed] 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.

[38] 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.

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,[46] with an intermediate stop at Nine Elms.

Today, all Northern line trains consist of 1995 Stock in the Underground livery of red, white and blue.

[58] Completely segregating the routes could allow 36 trains an hour on all parts of the line, increasing capacity by around 25%.

[56][58] 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.

[62] In 2018, plans to upgrade and rebuild Camden Town station were placed indefinitely on hold, due to TfL's financial situation.

[67] 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.

[68] 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.