Consultant Atkins has unveiled three business cases for high-speed rail alignments running north from London, although none include a major hub at Heathrow, as proposed by Arup last year.
Atkins transport planning managing director, Andy Southern, said: "A high-speed rail network should not only be viewed in terms of its benefit to relieving congestion on the rail network. The economic gain could also be extremely significant.
"Our modelling shows the impact would be felt by local communities as well as the business traveller choosing to go by rail rather than air. There is also potential to reduce overall carbon emissions from transport in the UK."
One route approximates to the existing East Coast Main Line, linking to existing services at York. The second approximates to the West Coast Main Line, with a Heathrow spur, to a point south of Birmingham to existing services.
A "full network" option includes high-speed routes through to Glasgow.
Consultant Arup revealed plans to extend HS1 from St Pancras via Heathrow and then north to Birmingham, Manchester and Glasgow last year "Arup recognises there is significant economic case in radically improving Heathrow's connectivity and linking the airport to the high-speed rail network," said Arup director Mark Bostock.
"We announced in December that we were looking into extending HS1 and our submission to the Department for Transport's consultation summarises our position."
In January Network Rail chief executive Iain Coucher admitted that new high-speed lines were under investigation A Network Rail spokesperson welcomed the Atkins report, and confirmed that high-speed lines were, "likely", but added:
"The challenge is to look at the developing and changing demographics and demand along particular corridors, and see what fits best. High-speed travel is likely to be one of a multitude of ultimate solutions."