Thameslink is a mainline route on the British railway network, running from Bedford, Luton, St Albans City, Peterborough, Welwyn Garden City, London Blackfriars and Cambridge via central London to Sutton, Orpington, Sevenoaks, Rainham, Horsham, Three Bridges, Brighton and East Grinstead.

Parts of the network, from Bedford to Three Bridges, run 24 hours a day, except on early Sunday mornings and during maintenance periods.

[2] The Thameslink Programme was a major £5.5 billion scheme to increase capacity on the central London section by accommodating more frequent and longer trains, and providing additional routes and destinations.

[7][10] As well as these services, during peak hours, several trains in each direction (approximately two trains per hour) run to/from Orpington (originating/terminating at either London Blackfriars, Luton, West Hampstead Thameslink or Kentish Town), all calling at Petts Wood in lieu of stations from St Mary Cray to Bat & Ball.

[7] In addition, there are seven trains per day in each direction that operate to/from East Grinstead (originating/terminating at either Bedford, West Hampstead Thameslink, St Pancras International or London Bridge), which, after calling at South Croydon, call at Sanderstead, Riddlesdown, Upper Warlingham, Woldingham, Oxted, Hurst Green, Lingfield and Dormans.

[11] There are also three extra trains throughout the evening peak hours that operate from Gatwick Airport to Bedford in that direction only, calling at various stations.

He also proposed a north-east to south-west such link (Liverpool Street to Charing Cross), all giving seamless, key, main-line connections.

[13] The Snow Hill Tunnel route remained open for cross-London freight trains until 1970, when the short section between Farringdon and Holborn Viaduct was closed.

The replacement route under Ludgate Hill was opened on 29 May 1990 by the Network SouthEast (sector of British Rail) concurrently with City Thameslink station, which was initially called St Paul's Thameslink but was renamed in 1991 to avoid confusion with St. Paul's station on the Underground (Central line), about 500 m (550 yd) away.

[20] Network Rail obtained planning permission and legal powers in 2006,[21] funding was secured in July 2007[22] and construction began in October 2007.

[23] Plans included rebuilding the station buildings at Farringdon (in conjunction with the Crossrail project) and West Hampstead Thameslink, total rebuild of London Bridge and Blackfriars stations, two new underground platforms at St Pancras International, a new tunnel north of St Pancras International to the East Coast Main Line to allow through services to Peterborough and Cambridge, and platform lengthening.

[clarification needed][26] Siemens Mobility was named preferred bidder on 16 June 2011, with the Desiro City train family.

[29] First Capital Connect acquired 23 four-coach Class 377 sets during 2009 on sublease from Southern, for the Thameslink route for additional capacity and to allow some of the Class 319 trains to be released for the Catford Loop service to Sevenoaks, now jointly operated with Southeastern under Key Output 0 of the Thameslink Programme.

[33] The invitation to tender for the Thameslink, Southern and Great Northern franchise was expected to be issued in October 2012, with the contract commencing in September 2013.

On 29 March 2012, the Department for Transport announced Abellio, FirstGroup, Govia, MTR Corporation and Stagecoach Group had pre-qualified to bid for the franchise.