Business chiefs have urged the Government to prioritise the building of Crossrail 2.
The proposed north-south rail line running across London between Hertfordshire and Surrey would "make a difference to the whole country", according to more than 70 bosses of firms in the region.
In a letter to Chancellor Philip Hammond and Transport Secretary Chris Grayling, they said 40% of the transport benefits will be outside the capital while 60,000 supply chain jobs and apprenticeships would be created across the UK.
The signatories warned that an already overcrowded regional transport network "threatens to stunt growth across the entire nation" as people struggle to get to work.
They also predicted that the journey time benefits of the HS2 high-speed rail project will be lost in queuing for onward services at Euston station.
Crossrail 2 would cost about £30 billion at 2014 prices.
London Mayor Sadiq Khan said: "Despite the project benefiting the whole of the UK, London has actually met half the funding cost.
"What we now need is for the Government to take note of these benefits and give us the green light to progress, for the good of the entire country."
The line would run as far north as Broxbourne in Hertfordshire and as far south as Epsom in Surrey, passing through central London via places such as Tottenham Court Road, Victoria, Chelsea and Clapham Junction.
Construction could start in the early 2020s and the railway could be open by 2033.
Research published by think-tank IPPR North last month found that the imbalance in transport infrastructure investment between London and the rest of England is set to get even worse.
More than half (54%) of spending on the country's transport networks is going to the capital, according to the analysis.
Some £1,943 is being spent per person in London on current or planned projects compared with just £427 in the North.
A Government spokesman said: "We have now received the Strategic Outline Business Case from Transport for London and will carefully consider it to ensure it is robust and includes a fair, sustainable and deliverable funding plan."