The station began life as part of the original route of the Charing Cross, Euston and Hampstead Railway (CCE&HR) (now part of the Northern Line). As the line here branched into two routes, to Hampstead and to Highgate, the design of the station was rather unusual, shaped like a V. The line to Hampstead (now the Edgware Branch) is under Chalk Farm Road; the line to Highgate (now the High Barnet branch) is under Kentish Town Road. With the narrowness of the roads above, and the necessity to keep directly beneath them to avoid having to pay compensation to landowners during construction, on both lines the northbound platform is directly above the southbound one.
At the apex of the V is a junction allowing northbound trains to take either of the branches north, and likewise allow the trains south from the branches to join the single southbound track. This resulted in four connecting tunnels. When the CCEHR and City & South London Lines were joined together after the City & South London Line became part of London Underground, a short extension from the Euston terminus of the City & South London was built to connect with each of the two northerly branches. This added another four tunnels to the junction, making it the most complex junctionTemplate:Full on the network; it is located beneath Camden High Street.
After the CCE&HR was combined with the City & South London Railway to form the Northern Line, southbound trains at either platform could use either route. This led to considerable confusion at the platforms, which gave no indication which would have the next train to the desired destination: nowadays an electronic departure board informs passengers as they enter the station.
The lifts and emergency stairs to the platforms were inside the vertex of the V, leading to four passageways, one to each of the platforms, with return passageways back to the lifts. This was replaced in later years, due to increasing congestion, with an escalator heading from the station building to a circulating area at the northern end of the platforms. This has only two pairs of parallel passageways, one for each branch, with a small side passage on each leading to the lower platforms. One set of the original lift passageways became part of the ventilation system, but the remaining one adds to the confusion of the station. The west façade was bomb damaged in 1940 and only partially rebuilt.
On October 19, 2003 the points at one of these connecting tunnels was the site of a derailment, which caused damage severe enough to close the line for over a week. No-one was badly hurt, as the train's movement was confined by the tunnel; 1995 tube stock carriages 51722 and 52722 were both seriously damaged by the impact. After the accident, trains were restricted to travelling either from the Edgware branch to the Bank branch or from the High Barnet branch to the Charing Cross branch. This continued for some time, and many considered whether it would be permanent (particularly as this would make managing the two branches through central London easier). However, full use of the junction was restored in March 2004.
A joint report by London Underground and its maintenance contractor Tube Lines concluded that poor track geometry was the main cause of the derailment and 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 so derail. The track at the derailment site is on a very tight bend in a tight tunnel bore, which prevent canting the track by dipping the height of one rail relative to the other, the normal solution in this sort of situation.
The station is particularly busy at weekends with tourists visiting Camden Market and Camden High Street, to the extent that entry is prohibited on Sunday afternoons to prevent overcrowding on the platforms. Congestion is predicted to get worse and entry may in the future be prohibited on Saturdays too. London Underground has therefore submitted plans for the station to be rebuilt.
Rebuilding the station would ease congestion and allow it to be kept open during normal hours. Step-free access for the disabled would also be provided. Furthermore, rebuilding would allow easier interchange and facilitate the planned segregation of Northern Line services in order to increase capacity on the Line as a whole.. In addition the general ambience of the station would be much improved.
There are plans to completely rebuild the platform area, demolishing the original liftshaft space and the current circulating area at the foot of the escalators and replacing them with a large two-level atrium, the upper level taking the form of a balcony, with various access points onto the platforms and escalator access between levels. This would be reached by an additional new set of escalators.
According to London Underground, achieving this reduction in congestion requires them to demolish the old station building. In addition, to provide both a new temporary exit during construction works, and to comply with modern safety standards, London Underground claims that it is necessary to demolish Buck Street Market, commonly known as Camden Market (one of five local markets), the Electric Ballroom, a church (housing two separate and growing congregations) and several houses and shops to the north of the station.
Apart from complaints about destruction of one of the ox-blood tiled station buildings (although there are over 20 of these Leslie Green designed buildings), there is a significant controversy over the demolition of the buildings to the north. Complaints particularly centred on London Underground's desire to replace the buildings with modern construction said to be out of place and out of scale with the remainder of Camden Town, together with complaints about the loss of the buildings and market themselves. This led to a public enquiry being held. In January 2004, consultants Arup published plans commissioned by Save Camden, a group of local market traders, for a remodelling that would preserve the majority of the threatened buildings, including the market. In 2005 Transport for London lost its appeal to the office of the Deputy Prime Minister and the scheme has been cancelled. It is not known when they will propose an alternative scheme, or what it will involve.
As one of only three stations where transfers between the Bank and Charing Cross branches are possible and the northern of the two junctions between them, Camden Town features a complex platform arrangement. Similar to its sister station of Kennington, the station has four platforms with cross-platform transfers available between branches.
However, unlike Kennington, trains do not terminate at Camden Town, thus there are no terminus platforms or a loop to turn around terminating trains. Instead, all northbound trains use either platform 1 or 3, with Edgware branch trains using the former and High Barnet branch trains using the latter. Likewise, all southbound trains use either platform 2 or 4.