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 and the jointly operated First Capital Connect Southeastern services, which were added to the franchise on 25 July 2015, with the Southern and Gatwick Express brands retained.

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.