Wikipedia carries an article about the station and its functions:
Highbury & Islington station is a London Underground and National Rail station in the London Borough of Islington in north London. It is served by the Victoria line, London Overground's East and North London Lines and First Capital Connect's Northern City Line.
On the Underground Victoria line the station is between Finsbury Park and King's Cross St. Pancras stations. The station is in Travelcard Zone 2.
To allow new four-car trains to run on the London Overground network, the North London Line between Gospel Oak and Stratford was closed between February and May 2010 while a new signalling system was installed and 30 platforms were extended. During this closure, the Highbury & Islington station ticket hall was extended and step-free access installed to all of the London Overground platforms. The North London Line reopened on 1 June 2010; however, the East London Line platforms did not open until March 2011, whilst the Western Curve was reinstated, linking the station to Dalston Junction and the rest of the East London Line.
The current station derives from two earlier stations. The first, which was on the same site, was an impressive Victorian-gothic building, with a drive-in forecourt, built in 1872 by the North London Railway (NLR).
The second station, on the opposite side of Holloway Road, was opened on 28 June 1904 by the Great Northern & City Railway (GN&CR) on its underground line, between Finsbury Park and Moorgate. This line and stations were operated by the Metropolitan Railway and its successors from 1913 until 1975 when the line, by then called the Northern City Line, was transferred to British Rail. The route is now operated by First Capital Connect. Trains do not currently serve the Northern City Line in the late evening or at weekends, running instead to London King's Cross.
The NLR station was damaged by a V-1 flying bomb on 27 June 1944, however, its main building remained in use until it was demolished in the 1960s during the building of the Victoria line. The original westbound platform buildings remain, as does a small part of the original entrance to the left of the present station entrance.
Today's single-storey structure was built in the 1960s for the opening of the Victoria line on 1 September 1968 and is the entrance for all lines. When the escalators to the deep level platforms were opened the GN&CR station building was closed. Its disused entrance remains and was refurbished externally in 2006 - it houses signalling equipment for the Victoria line.
The Victoria line was built to give as many interchanges as possible with Underground and British Rail lines, with, wherever possible, cross-platform connection between different lines heading in the same direction. To this end at Highbury & Islington the northbound Northern City Line platform was reallocated to the southbound Victoria line to give a direct link between the two southbound platforms; a new northbound platform was constructed for each line; the northbound running NCL tunnel was diverted to its new platform; and the southbound Victoria tunnel was joined to the old northbound NCL tunnel.
Former train operating company Anglia Railways ran services known as London Crosslink from Norwich to Basingstoke via Stratford, which called at Highbury & Islington. This service operated from 2000 until 2002.
Highbury & Islington has a total of eight platforms, divided between those that are on the surface and those that are deep level:
Platform 1 - East London Line services to Crystal Palace
Platform 2 - East London Line services to Clapham Junction
Platform 3 - Northbound Victoria line
Platform 4 - Northbound Northern City Line
Platform 5 - Southbound Victoria line
Platform 6 - Southbound Northern City Line
Platform 7 - Westbound North London Line
Platform 8 - Eastbound North London Line
Platforms for North London Line (NLL) and East London Line (ELL) services are operated by London Overground. On 1 June 2010, following the temporary closure of the route from February 2010 to May 2010, NLL services were rerouted to the newly built platforms 7 and 8 for the AC lines, which replaced the old "special use" platform. Platforms 1 and 2, which previously served the NLL route on third rail lines were closed from February 2010 for reconstruction and reopened in 2011 for the ELL services. The change of platforms allows ELL services to operate without having to cross over NLL tracks.
Platforms 3 to 6 are deep-level platforms. Platforms 3 and 5 are used for services on the Victoria line; 4 and 6 are used for First Capital Connect services on weekdays only.
London Buses Routes 4, 19, 30, 43, 271, 277, 393, Night Routes N19 and N41 serve the station.