The search has begun to find a contractor to build the £260m Barking Riverside Overground extension in east London.
Transport for London has now formally opened bidding for the 4.5 km rail line, which will serve London’s largest planned housing development.
TfL is seeking expressions of interest from contractors, with a formal invitation to tender expected to be launched in 2017.
Construction is planned to begin in late 2017, with the line and a new Barking Riverside station to be fully operational by 2021.
The bidding opens nearly six months after TfL approved £172m of funding for the line, paving the way for construction to begin on the residential project.
Original developer Bellway Homes had planned to build 10,800 homes on the site in 1994 but these proposals were shelved because of the lack of transport links.
TfL submitted a Transport and Works Act Order to the transport secretary in March, with a decision expected later this year.
The Barking Riverside development will see more than 10,800 homes built along with a new school, retail outlets and leisure facilities.
It will now be developed by L&Q after the company bought a 51 per cent stake in the project from Bellway Homes.
Bellway will remain a development partner on the project.
L&Q also agreed to commit £70m to the construction of the Overground extension and new station as part of the deal.
A section 106 agreement meant up to 1,500 homes could be built ahead of construction of the Overground extension, but only 800 homes have been delivered so far, according to Barking & Dagenham Council.
Deputy mayor of London for transport Val Shawcross said: “The extension of the Overground will be key to the success of the exciting new developments at Barking Riverside, with a new station built alongside 10,800 new homes, shops and restaurants, and new school and health services.
“As we have seen in other parts of London, the extended Overground line will be an integral part of ensuring regeneration benefits everyone in the community, with excellent transport links to the rest of London and the creation of new homes and jobs.”