A public consultation on the multi-billion pound, cross-London Crossrail 2 scheme was launched today.
The public is being asked to give its views on two options for Crossrail 2 which will complement the £15 billion west-east, cross-London Crossrail 1 scheme due for completion in 2018.
Transport for London (TfL) has proposed either a metro option for Crossrail 2 or a regional option.
The metro option would be an underground railway operating between Wimbledon in south west London and Alexandra Palace in north east London.
This would relieve congestion on trains and platforms on the Tube's Northern, Piccadilly and Victoria lines.
The regional option would be a combined overground and underground railway from as far north as Cheshunt in Hertfordshire, running through central London to as far south as Epsom and Chessington in Surrey.
Needing public and private financing, Crossrail 2 could be completed around 2030.
London Mayor Boris Johnson said today: "Crossrail (1) is set to revolutionise travel in the capital, and with a predicted 10 million people expected to be living in London by 2031, pressing ahead with the next stage of the plan, Crossrail 2, is quite simply essential.
"In order to support this great capital's bright future, we need to think ahead and not wait until our transport arteries are clogged up and restricting jobs and growth. That's why I am sending out a rallying cry for the public's support for this consultation. Come and share your views so we can progress with Crossrail 2 as soon as possible."
The consultation will seek the views of people in London and south east England and will run from today to 2 August.
Speaking at Wimbledon train station today as he launched the consultation, Mr Johnson said he was "confident" about securing investment for the project, which is estimated to cost between £9 and £12 billion.
The Mayor of London said he hoped Crossrail 2 would be completed by 2028 to prevent train conditions which are "unfit for livestock".
"I think people massively support new investment in infrastructure," he said.
"They know the strain that London is under and will face in the next few years. We're going to increase our population by about a million between now and 2021.
"Just since I've been mayor, the population of London has risen by anything between 400,000 and 600,000 depending on which census you believe. That's a huge increase just in four or five years. It puts massive pressure on the rail networks."
He added: "In the end we can not be carrying people around in conditions that the European Union would condemn as unfit for livestock. We've got to have enough transport capacity."