[2] The original Thameslink rail network was created by joining the electrified network south of the Thames with the then recently electrified line between Bedford and St Pancras to the north via the Snow Hill tunnel, allowing passengers to travel between stations to the north and south of London, including Bedford, Luton Airport, Gatwick Airport and Brighton, without changing trains or using the London Underground.

[4] Passenger traffic between destinations in north and south London served by Thameslink services quadrupled after the first year of operation.

[5] The success of this initial project encouraged British Rail to develop proposals to extend the network, and increase the frequency of service.

Railtrack applied for Transport and Works Act 1992 powers on 21 November 1997,[7] but two months later London and Continental Railways (LCR), a company created to build the High Speed 1 railway between London and the Channel tunnel, announced that it would require a further direct government grant of £1.2 billion (1995 prices) to finance the rail link;[8] and the Thameslink Programme relied on the construction of a 'concrete box' provided by this other project to house the new Thameslink sub-surface station underneath St Pancras station.

The Government and LCR did however reach agreement in June 1998[8] allowing the construction of High Speed 1 and also the associated works required for the Thameslink programme to proceed.

During this period Railtrack carried out an extensive public consultation exercise, which resulted in the revision of the original proposals; it then submitted a Supplementary Order on 29 September 1999.

[citation needed] Network Rail revised the original proposal and submitted it along with an updated Environmental Statement dated 14 June 2004.

[14] During this period the Strategic Rail Authority was abolished by the Railways Act of 2005, and the Department for Transport (DfT) took over funding responsibility for the project in July 2005.

[15] The second public inquiry took place between September and December 2005,[16] and the Inspector completed the report in February 2006,[17] submitted to the DfT for consideration.

In October 2006 the DfT published the second report, declaring that the Inspector was satisfied that the deficiencies of the previous proposals had been resolved, and recommending that the project be approved.

The drawback to this idea was that the planned routes to Cambridge, King's Lynn, Ashford (Kent), Littlehampton and Eastbourne would have been abandoned and the improvements would have only benefited London and commuter belt towns within 15 to 20 miles (20 to 30 km) of the city.

[21] On 24 July 2007, the Secretary of State for Transport, Ruth Kelly, formally announced that the Government was fully committed to funding the Thameslink Programme.

[23] With the downturn in the economy there was some doubt that phase 2 of the project would be completed in its entirety, but on 25 November 2010 the Secretary of State for Transport (Philip Hammond) confirmed that no cutbacks would be made.

However, project completion would be delayed until 2018 as the schedule was already difficult to achieve and because cost savings could be made if the level of concurrent activity was reduced.

[29] This involved signalling works and alterations to the tracks and overhead line equipment between Farringdon and City Thameslink to allow the merged services to operate.

Works at Farringdon also allowed for the east–west Crossrail route (which opened as the Elizabeth line in May 2022), while Blackfriars gained a new entrance on the south bank of the Thames.

New permanent way (track level, in this case including both plain rail and junctions) and overhead line equipment were laid out in the new Canal Tunnels just north of St Pancras between the Thameslink route and the East Coast Main Line,[36] opening up the Thameslink network to new destinations north of London.

[39] As part of the project, a purpose-built fleet of 55 12-car and 60 8-car Class 700 trains (equivalent to 1,140 carriages) is now operating across the network (see the "Rolling stock" section below).

[48] Two new low-level platforms at St Pancras International which accept 12-carriage trains replaced the old King's Cross Thameslink station[49] and opened on 9 December 2007.

London Bridge station has undergone a major transformation to accommodate Thameslink trains during the peak period and achieve many other benefits.

[51] New retail facilities have been built into the existing western arcade, which has been re-opened and extended to link the Underground station and Joiner Street.

These vary from quite major works such as at Luton, where new bridges had to be installed, and West Hampstead, where a new station footbridge has been built, to relatively straightforward platform extensions.

[58] Widening of the existing viaduct west of Stoney Street was to the north to avoid affecting the Hop Exchange building to the south.

Thameslink trains in both directions now cross over the Kent lines on a new bridge, meeting the existing alignment just north of Jarrow Road.

Passing under the Regents Canal, the tunnels connect the East Coast Main Line near Kings Cross to the Thameslink route, with services from Cambridge and Peterborough that started in February 2018.

The OHLE (overhead line equipment) system has been extended from Farringdon to City Thameslink and was commissioned in December 2009,[72] Combined with a new crossover in Snow Hill tunnel between the two stations, this allows southbound trains to be turned back should they fail to change from AC overhead line traction current to DC third rail.

[78] As part of consultations, the proposed timetable received further tweaks in September 2016[79] and June 2017:[80] In 2009, the Thameslink fleet consisted of 74 dual-voltage Class 319s.

While the Thameslink Rolling Stock Project (TRSP) will provide a brand-new fleet, the timescales involved were such that interim solutions were required.

Network Rail had planned to terminate Sutton Loop Thameslink trains at Blackfriars station, rather than have them continue through central London as at present.