London & South Eastern Railway Limited, trading as Southeastern, is a British train operating company owned by Govia that operates passenger rail services in South East England. It is the key operator of commuter and regional services in South East London and Kent, and also serves parts of East Sussex.

Southeastern trains operate on three main routes: the Southeastern Main Line from London Cannon Street and London Charing Cross to Dover via Sevenoaks; the Chatham Main Line between London Victoria and Dover/Ramsgate via the Medway towns; and High Speed 1 from London St Pancras.

Southeastern began operations on 1 April 2006 as the franchisee for the new Integrated Kent franchise (IKF), replacing the publicly owned South Eastern Trains on the former South Eastern franchise. Southeastern has received a number of extensions since, with the franchise scheduled to end on 1 April 2020 until a new contract was agreed on 30 March 2020, running for up to two years. The Southeastern franchise was terminated on 16 October 2021 and transferred to the SE Trains, after an investigation into £25 million of taxpayer money being unaccounted for.

SE Trains will serve the main London stations of Cannon Street, Charing Cross, London Bridge, St Pancras, Victoria and Waterloo East. The network will have a route mileage of 540, with 179 stations.