The East London line extension (ELLX) project was a British railway engineering project in London, managed by Transport for London. The project involved extending the East London Line (formerly part of the London Underground network) and making it part of the mainline London Overground network. This was done by re-opening sections of disused railway line and by converting track electrified by the third-rail system, signalling, lineside signage and communication systems, etc. to mainline standards. New rolling stock was introduced and four new stations built along the route, with a fifth scheduled to be added in the future at New Bermondsey.
The work, costing around £1 billion, began in 2005 and is taking place in two phases. The first phase was completed on 23 May 2010 with a service from Dalston Junction via the historic Thames Tunnel under the River Thames and along part of the Brighton Main Line to West Croydon and Crystal Palace. Trains also go to New Cross, where they make connection with the South Eastern Main Line. On 28 February 2011 the line was connected at its northern end to the North London Line at Highbury & Islington. In the second phase of the project, due in December 2012, a branch is being connected to the South London Line enabling services to run to Clapham Junction Station. In the second phase of the project, on 9 December 2012 a branch was connected to the Inner South London Line enabling services to run to Clapham Junction