London's mayor has pledged a £900m rail link will be completed by 2010 - boosting the bid for the Olympic Games.
The East London Line would serve the proposed Olympic Park
Ken Livingstone said work on the East London Line extension - from Dalston to Croydon via Crystal Palace - would begin in 2005 and finish by June 2010.
The link is seen as crucial in assuring the International Olympic Committee London's transport infrastructure could cope with the demands of the Games.
It would provide a metro service with trains running at Tube-style frequency.
The announcement comes days after responsibility for the project was officially transferred to the mayor's transport authority Transport for London (TfL).
Mr Livingstone was at the unveiling of a Docklands Light Railway train painted in the colours of the 2012 Olympics bid on Tuesday.
He said: "We have been talking about this for 20 years which is why I
think it needed the spur of the Olympics to push it finally to that point where
we have the money to build it and go ahead."
Lord Coe, who joined the mayor at Canary Wharf Station, east London, said: "This is an incredibly important part of our public campaign. It shows just how much London is behind the 2012 Olympic bid."
Final detailed proposals of London's bid were flown to the IOC headquarters in Lausanne, Switzerland, on Monday.
London will find out if it has successfully beaten off competition from Paris, Madrid, New York and Moscow on 6 July 2005.