Encouraging travellers to switch from cars and airlines to inter-city trains brings no benefits for the environment, new research has concluded.
Challenging assumptions about railways' green superiority, the study finds that the weight and fuel requirements of trains have increased to the point where rail could become the least energy-efficient form of transport.
Engineers at Lancaster University said trains had failed to keep up with the motor and aviation industries in reducing fuel needs.
They calculate that expresses between London and Edinburgh consume slightly more fuel per seat (the equivalent of 11.5 litres) than a modern diesel-powered car making the same journey.
The car's superiority rises dramatically when compared with trains travelling at up to 215mph.
The Government is considering such a railway to provide extra north-south capacity in the next decade.
Assuming the continuing dominance of fossil fuel-based electricity, the study indicates that suitable French-style rolling-stock would require twice as much fuel per seat as a Volkswagen Passat, and more than a short-haul aircraft.
Prof Roger Kemp, who led the research, said that in its efforts to improve performance after privatisation, the rail industry had "taken its eye off the ball" environmentally.
Virgin's SuperVoyager rolling-stock is estimated to be 40 per cent heavier per seat than the ageing 125s it replaced.
Tilting Pendolino trains, due to come into full operation between London and Manchester in the autumn, are reckoned to weigh more per seat than the forthcoming Airbus A380 double-decker.
Roger Ford, of Modern Railways magazine, said one reason for declining energy efficiency was the impact of health and safety and disability access regulations.
The introduction of crumple zones, disabled lavatories and seating rules for trains travelling over 100mph had added weight and reduced capacity.
"I know this will generate howls of protest, but at present a family of four going by car is about as environmentally friendly as you can get."
The research casts doubt on the Government's conviction that boosting inter-city rail travel will help it meet its environmental targets.
It also undermines the case of those who oppose airport expansion in favour of improved high-speed rail links.
Friends of The Earth expressed surprise at the findings, which it said were not in line with previous studies.
Tony Bosworth, its transport campaigner, said: "Cars cause congestion, disrupt communities and are much less safe than trains.
"The main problem is not long-distance travel but the 25 per cent of car journeys which are less than two miles. Those are the least efficient and often the most polluting."