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.

The railway system in Great Britain is the oldest railway system in the world. The first locomotive-hauled public railway opened in 1825, which was followed by an era of rapid expansion. Most of the track is managed by Network Rail, which in 2017 had a network of 9,824 miles (15,811 km) of standard-gauge lines, of which 3,339 miles (5,374 km) were electrified. In addition, some cities have separate metro, light rail and tram systems, among them the historic London Underground and the Glasgow Subway. There are also many private railways, some of them narrow-gauge, which are primarily short lines for tourists. The main rail network is connected with that of continental Europe by the Channel Tunnel and High Speed 1, opened in 1994 and 2007 respectively.