A preferred route for a proposed £20bn Crossrail 2 scheme, running north-south through London, has been identified by mayor Boris Johnson.
A second Crossrail scheme is vital to support the capital's growth, the mayor claimed.
If approved by the government, the line could be transporting up to 90,000 people into central London in the morning peak by 2030.
Crossrail 2 would run from Cheshunt in Hertfordshire to Epsom in Surrey.
It would pass through central London via places including Tottenham Court Road, Victoria, Chelsea and Clapham Junction.
If proposals are approved, construction could begin in 2017.
Setting out the case for the line in a speech at the City Age conference, the mayor said: "With London's population soon to surpass its previous 1939 peak of 8.6 million and with more people travelling by Tube and rail than ever before, we need additional rail capacity to support future growth.
"For the capital to remain globally competitive, there needs to be continued investment in our transport network and that's why we have to get cracking on planning for Crossrail 2."
The scheme would complement the £14.8bn east-west Crossrail scheme, currently being built and scheduled to be operational by 2018.
Supporters say it will slash journey times across London, with a journey from Kingston, in south-west London, to Tottenham Court Road being completed in 29 minutes - 17 minutes faster than today.
Those travelling between Dalston, in north-east London and Tottenham Court Road would have an eight-minute journey - 19 minutes faster than today.
Mr Johnson said he was confident the private sector could, in the right circumstances, contribute to well over half the cost of Crossrail 2.
Opportunities for consultation
Labour Assembly Member Val Shawcross said the announcement of a preferred route was "a big step forward" but added: "What we need to see now is the fully worked-up funding package which will make this project a reality."
Despite broad support for a new rail link in Chelsea, it is one area where there is concern over the positioning of a new station.
The current proposed location for the station would be further east near the fire station on the King's Road and received the majority of support in this summer's consultation.
The consultation document says, over the course of 2015, there will be further work on the consideration and assessment of options and a number of opportunities for more detailed consultation.