Thameslink is a 50-station main-line route in the British railway system running 225 km (140 mi) north to south through London from Bedford to Brighton, serving both London Gatwick Airport and London Luton Airport. It also incorporates a suburban loop serving Sutton and Wimbledon. It opened as a through service in 1988 and by 1998 was severely overcrowded, carrying more than 28,000 passengers in the morning peak. The Thameslink Programme is a major £5.5 billion scheme to extend the service to a further 100 stations and to greatly increase capacity on the central London section to accommodate more frequent and longer trains. This scheme, scheduled for completion in 2018, is well under way.

Most of the route is over the Brighton Main Line and the southern part of the Midland Main Line. There is also a suburban loop through Sutton and Wimbledon.

The route through central London is via St Pancras International for connections to Eurostar and the East Midlands; Farringdon, for London Underground's Circle, Metropolitan and Hammersmith & City lines, and Crossrail from 2018; City Thameslink, which replaced the demolished Holborn Viaduct and also has a southern entrance serving Ludgate Circus; Blackfriars, for other rail services and the Underground District and Circle lines; and London Bridge, for links to a number of other lines. King's Cross Thameslink on Pentonville Road closed on 8 December 2007.