Thameslink is a fifty-station franchise in the British railway system running 225 km north to south across London from Bedford to Brighton through the Snow Hill tunnel. It is a significant commuter route and serves the airports at Gatwick and Luton. There are around 40 million passenger journeys on the system annually.

The tunnel was re-opened after fifty years in 1988 and the network in May 1990. The northern part of the network replaced the "Bedpan" service from Bedford to St Pancras station, and uses the existing Midland Main Line. The central London stations are King's Cross Thameslink station on Pentonville Road close to King's Cross rail station; London Bridge rail station which links to a number of other franchises; Farringdon rail station which links into the London Underground Circle and Metropolitan lines; City Thameslink rail station, (formerly St. Paul's Thameslink station; the name was probably changed to avoid confusion with St. Paul's tube station which is a considerable distance away), which replaced the demolished Holborn Viaduct, and Blackfriars.

In the south there are two branches. The "main line" runs through London Bridge and then on to East Croydon then down to Brighton. A second branch has a more convoluted history. Initially the line ran via the Elephant and Castle then via Streatham to West Croydon. Although this route, still used by other train services, comes close to the "main line", it never relinks with it. After West Croydon the line ran through Carshalton Beeches to Sutton then to Epsom, Leatherhead, Eppingham Junction and finally terminating at Guildford. However this route crossed the commuter networks of what were to become several different rail companies and the onset of rail privatisation made the route increasingly difficult to maintain. In 1994(?) the second branch was cut back to West Croydon. Then in 1995(?) a major overhaul occurred when the route was changed completely. West Croydon was abandoned by Thameslink and instead a new route to Sutton was opened up over existing track through Mitcham Junction with the line then continuing on a loop up to Wimbledon and then rejoining itself south of Elephant and Castle.

The franchise is currently run by the train operating company (TOC) Govia, a joint venture between Go-Ahead Group and Keolis (previously Via-GTI, renamed following its acquisition by SNCF). Govia acquired the franchise from March 2, 1997 for seven years and a day.

The Thameslink rolling stock is relatively modern, built by BREL in 1987 and 1988. The Class 319 trains are electrically powered dual-voltage four-car units. They use 25kV AC overhead power north of Farringdon and 500 Volt DC third rail to the south. Thameslink has 72 of these trains which are rated to hold 284 or 314 passengers.

As part of the Thameslink 2000 project, 800 million will be spent expanding the network to 169 stations, spreading northwards to Peterborough, Cambridge and King's Lynn, and south to Guildford, Eastbourne, Ashford and Dartford. King's Cross Thameslink will be closed and replaced by a new facility under St Pancras station. The scheme was suggested in 1991, approved in 1999 and moves towards construction began in 2001. The planning inspector's report of July, 2002 said there was a strong case for the project, but three planning issues remained to be resolved. A revised schedule of January 2003, subject to plannning issues, suggests work under St Pancras station in 2004, completion of "Phase 1" for increasing the capacity of the northern routes by 2007 or 2008, and full through-running following remodelling of London Bridge station by 2012.

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