Question: What is the former name of the rail service to which the Northern City Line belongs?

full_context: Document 1: West Anglia Great Northern: West Anglia Great Northern was a train operating company in England owned by National Express, that operated the West Anglia Great Northern franchise from January 1997 until March 2004 and the Great Northern franchise from April 2004 until March 2006.
Document 2: Govia Thameslink Railway: Govia Thameslink Railway (GTR) is a train operating company that operates the Thameslink, Southern and Great Northern (TSGN) rail franchise in England. Within the franchise, GTR runs the Thameslink, Great Northern, Southern and Gatwick Express airport services.
Document 3: Stevenage railway station: Stevenage railway station serves the town of Stevenage in Hertfordshire, England. The station is 27 mi north of London King's Cross on the East Coast Main Line. Stevenage is managed and served by Great Northern. It is also served by Hull Trains and Virgin Trains East Coast.
Document 4: Essex Road railway station: Essex Road railway station is a National Rail station in Canonbury in the London Borough of Islington. It is on the Northern City Line between Old Street and Highbury & Islington , 1 mi down-line from , and is in Travelcard Zone 2. The station is at the junction of Essex Road, Canonbury Road and New North Road, with the present entrance on Canonbury Road. Operated by Great Northern, it is the only deep level underground station in London served solely by National Rail trains. Between 1933 and 1975 the station was operated as part of the London Underground, as a short branch of the Northern line. Between 1922 and 1948 the station name was Canonbury & Essex Road. The name reverted to the original form in 1948.
Document 5: East Coast Main Line: The East Coast Main Line (ECML) is a 393 mi major railway link between London and Edinburgh via Peterborough, Doncaster, Wakefield, Leeds, York, Darlington and Newcastle, electrified along the whole route. Services north of Edinburgh to Aberdeen and Inverness use diesel trains. The main franchise on the line is operated by Virgin Trains East Coast.
Document 6: Northern City Line: The Northern City Line is a branch railway and commuter line in England, which runs from Moorgate to Finsbury Park in London with services running beyond. It is part of the Great Northern Route services and operates as the south-eastern branch of the East Coast Main Line (ECML). It is underground from Moorgate to Drayton Park in Highbury from which point it runs in a cutting until joining the ECML south of Finsbury Park. Its stations span northern inner districts of Greater London southwards to the City of London, the UK's main financial centre. Since December 2015 the service timetable has been extended to run into the late evenings and at weekends, meeting a new franchise commitment for a minimum of 6 trains per hour until 23:59 on weekdays and 4 trains per hour at weekends.
Document 7: West Coast Main Line: The West Coast Main Line (WCML) is one of the most important railway corridors in the United Kingdom, connecting the major cities of London, Birmingham, Glasgow, Liverpool and Manchester. It is one of the busiest mixed-traffic railway routes in Europe, carrying a mixture of intercity rail, regional rail, commuter rail and rail freight traffic. The core route of the WCML runs from London to Glasgow, with branches diverging to Birmingham, Manchester and Liverpool, totalling a route mileage of 700 mi . Services from London to North Wales and Edinburgh also run via the WCML; however the main London-Edinburgh route is the East Coast Main Line. In addition, several sections of the WCML form part of the suburban railway systems in London, Birmingham, Manchester and Glasgow, with many more smaller commuter stations, as well as providing links to more rural towns.
Document 8: Great North Road (Great Britain): The Great North Road was the main highway between London and Scotland. It became a coaching route used by mail coaches travelling between London, York and Edinburgh. The modern A1 mainly parallels the route of the Great North Road. Coaching inns, many of which survive, were staging posts providing accommodation, stabling for horses and replacement mounts. Nowadays virtually no surviving coaching inns can be seen while driving on the A1, because the modern route bypasses the towns in which the inns are found.
Document 9: Thameslink, Southern and Great Northern franchise: Thameslink, Southern and Great Northern (TSGN) is a management contract for the provision of passenger services on the Thameslink and Great Northern routes to Bedford , Luton , Peterborough , King's Lynn , Cambridge , London King's Cross , London Moorgate, Sutton, Wimbledon and Brighton, as well as the whole Southern network (including the Gatwick Express) and the jointly operated First Capital Connect Southeastern services (for example the Kentish Town - Sevenoaks via Catford ), which were added to the franchise on 25 July 2015, with the Southern and Gatwick Express brands retained.
Document 10: Northern line extension to Battersea: The Northern line extension to Battersea is an extension of the London Underground Northern line currently under construction from Kennington to Battersea in South West London, terminating at the redeveloped Battersea Power Station. The extension will form a continuation of the Charing Cross branch of the line. Two new underground stations will be built at Battersea Power Station and Nine Elms which will serve a new embassy district including, amongst others, the United States embassy and the Dutch embassy.
Document 11: City Line (Merseyrail): The City Line is the name given to local rail routes out of Liverpool Lime Street railway station. It appears on maps of the Merseyrail network as red, and covers the Liverpool–Wigan line as well as the two routes of the Liverpool–Manchester lines. Although it is branded under the Merseyrail name, the routes are operated by Northern on behalf of Merseytravel rather than by the Merseyrail franchise.
Document 12: Great Northern Route: The Great Northern Route (formerly known as Great Northern Electrics) is the name given to suburban rail services run on the southern end of Britain's East Coast Main Line and its associated branches. Services operate to or from London King's Cross and Moorgate in London. Destinations include Hertford North , Welwyn Garden City , Stevenage , Peterborough , Cambridge and King's Lynn . Services run through parts of Greater London, Hertfordshire, Bedfordshire, Cambridgeshire and Norfolk.
Document 13: Drayton Park railway station: Drayton Park railway station is a National Rail station in Highbury, in the London Borough of Islington. It is on the Northern City Line between Highbury & Islington and Finsbury Park stations, 2 mi down-line from ; it is in Travelcard Zone 2.
Document 14: Thameslink and Great Northern: Thameslink and Great Northern are the brand names used by the Govia Thameslink Railway train operating company on the Thameslink and Great Northern routes of the Thameslink, Southern and Great Northern franchise, previously operated by First Capital Connect.
Document 15: Highbury &amp; Islington station: Highbury & Islington is a London Underground and National Rail interchange station in the London Borough of Islington, north London. It is served by the London Underground's Victoria line, London Overground's East and North London Lines, and Great Northern's Northern City Line.
ground_truth: Great Northern Electrics