Much of the original route is over the Brighton Main Line (via London Bridge) and the southern part of the Midland Main Line, with a suburban loop through Sutton and Wimbledon. A branch via the Catford Loop Line to Sevenoaks was added in 2012. Sections to Peterborough on the East Coast Main Line, Cambridge via the Cambridge Line, Horsham on the Arun Valley line and Rainham via Greenwich were added in 2018. East Grinstead and Littlehampton are also served in peak hours.