A £13bn alternative to the Crossrail scheme which would link east and west London has been unveiled.
The team behind Superlink say it would carry four times as many people and take pressure off the Tube.
Planners say it would also be more far reaching and benefit areas outside London including Buckingham, Berkshire, Hampshire and Essex.
But rail industry groups have criticised the scheme, saying it could delay the planned rail link.
Railway Forum director general Adrian Lyons said Crossrail had reached a critical point in its development and should go forward.
"To now suggest that it should not go ahead and that we should place our faith in a concept that is, at best, in the earliest stages of development and will cost more is just
not an option," he said.
Serve Stansted airport and new housing development in south-east London
Relieve overcrowding on the Tube and national rail services
Directly service many key centres in South East England
Provide better rail access to Heathrow airport including links from South
Take traffic off the motorway network, particularly the M25
Release capacity on existing rail lines to allow improved local services
Jeremy Candfield, director general of the Railway Industry Association, said: "Crossrail has been studied for years, with many different options evaluated
"Stopping to consider yet another set of proposals would just mean more delay."
But Dr John Prideaux, a former senior British Rail executive who chairs Superlink,
said: "We think that the construction of a new east-west railway is too
big an opportunity to be allowed to go wrong.
Dr John Prideaux chairs Superlink
"The Government has backed the 1980s Crossrail scheme because it thought
that it was the only game in town, even though its supporters admit they don't
know who will pay the £10bn cost.
"Our scheme serves the planning needs and traffic patterns of the 21st century. That it why it will work and, above all, can be funded."
The first stage of Superlink would join Cambridge and Stansted, via Canary Wharf.
Both schemes would use a long tunnel through central London but the new scheme would build further strategic lines across the capital for faster journeys into the home counties.
Neither of the routes would be ready for the 2012 Olympic Games, if London's bid were successful.