The Thameslink Programme, originally Thameslink 2000, was a £6 billion project in south-east England to upgrade and expand the Thameslink rail network to provide new and longer trains between a wider range of stations to the north and to the south of London. The development facilitated new cross-London journeys, which means that passengers no longer have to change trains in London. Work included platform lengthening, station remodelling, new railway infrastructure, and new rolling stock. The project was originally proposed in 1991 following the successful introduction of the initial Thameslink service in 1988. After many delays, planning permission was granted in 2006 and funding was approved in October 2007. Work started in 2009 and was completed on 18 September 2020, although trains over the new routes began running in 2018.

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. The network opened as a through service in 1988, with severe overcrowding by 1998, carrying more than 28,000 passengers in the morning peak. All the services are currently operated by Govia Thameslink Railway. Parts of the network, from Bedford to Three Bridges, run 24 hours a day, except on early Sunday mornings and during maintenance periods.