It can be greener to drive than catch the train, according to a rail industry study which reveals that trains are losing their environmental advantage.

Modern diesel-powered trains are so polluting that a family of three or more would be responsible for at least double the carbon dioxide emissions on many routes when travelling by rail compared with driving in a typical medium-sized car.

The study concludes that the Virgin Voyager, the most advanced diesel train on the network, has the highest emissions of any British train and that its performance compared with cars is steadily worsening as motor manufacturers improve efficiency.

The study, commissioned by the Rail Safety and Standards Board, urges the Government to electrify key sections of the rail network to allow greener electric trains to replace diesel ones. On several long-distance routes, such as London to Hull, diesel trains run long distances under electric wires because short stretches of track have not been electrified.

Only 40 per of Britain’s rail network is electrified, the lowest proportion of any large European country. The best-performing electric trains are operated by GNER between London and Edinburgh and emit only 40g of CO2 per passenger-kilometre (g/pkm) compared with 112g/pkm for Voyagers.

By 2022, more efficient power generation will have reduced the emissions of the GNER trains to 28g/pkm. But the emissions of the Voyagers, which are only five years old and are due to remain in service until after 2030, will be unchanged. On present trends, emissions from the average car will have been reduced from 131g/pkm to 98g/pkm by 2022.

The numbers are based on the existing average passenger loads on cars and trains. Cars carry an average of 1.6 people and, across the whole day, a third of train seats are occupied.

The study says: “As the efficiency of cars progressively increases, the difference in emissions between cars and high-performance trains will narrow and it will be increasingly difficult to make an environmental case for transferring people on to diesel-powered railways.” Its author, Roger Kemp, Professor of Engineering at Lancaster University, said that the Government should focus on attracting business travellers, rather than families, to rail. “It’s not politically correct to say so, but the Government is better off encouraging families into low-emission cars and getting business people, who tend to travel alone in large cars, to catch the train.”

Professor Kemp said that he was sceptical about the experimental running of trains on biodiesel, made from plants. “I’m very doubtful of the claims made for biofuels because the overall CO2 can be even greater once you take into account what is emitted in production.”

He said that modern trains tended to be less efficient than older ones because they were much heavier. Safety regulations have added to the weight by requiring more robust bodies and crumple zones. New trains also carry more equipment, such as air-conditioning and motors for sliding doors, and have space-consuming lavatories for disabled passengers. Britain’s long-distance trains typically weigh more than a tonne per seat. By contrast, Japan’s bullet trains weigh only 500kg per seat as they are made using lighter, more advanced materials.

The Government is expected to address the environmental challenge facing the railways in a 30-year strategy being published this month. Ministers have already admitted that some trains on rural lines, such as the diesel Sprinter, are less efficient than 4x4s because they are often almost empty. Douglas Alexander, when he was Transport Secretary, said last year: “If ten or fewer people travel in a Sprinter, it would be less environmentally damaging to give them each a Land Rover Freelander and tell them to drive.”

An official at the Department for Transport said that the strategy would not set specific targets on electrifying more of the network because of uncertainty over how much electricity would be generated in future from low-emission sources.

Some companies which operate electric trains, such as Virgin West Coast and C2C, carry systems that allow them to capture and reuse the energy usually lost during braking.