Greenhouse gas emissions generated by trains are much more higher than those produced by an airplane, a new study has said.
Unlike previous studies on transport emissions, this one looks beyond what is emitted by different types of car, train, bus or plane while their engines are running and includes emissions from building and maintaining the vehicles and their infrastructure, as well as generating the fuel to run them.
Including these additional sources of pollution more than doubles the greenhouse gas emissions of train travel.
The emissions generated by car travel increase by nearly one third when manufacturing and infrastructure are taken into account.
In comparison to cars on roads and trains on tracks, air travel requires little infrastructure. As a result, full life-cycle emissions are between 10 and 20 per cent higher than "tailpipe" emissions.
According to a report in New Scientist, Mikhail Chester and Arpad Horvath of the University of California, Berkeley, included in their calculations data on the "life expectancy" of each component of each mode of transportation, such as the tracks used by a train and the airports used by aircraft.
They calculated the total "travel kilometers" each component allows and how many tonnes of greenhouse gases were emitted to build and maintain each component.
This allowed them to calculate the component's emissions per kilometre travelled, for each mode of transport per kilometre for each traveller on board.
Cars emitted more than any other form of transport with the notable exception of off-peak buses, which often carry few passengers.
Passengers on the Boston light rail, an electric commuter train, were found to emit as much or marginally more than those on mid-size and large aircraft. This is because burning fossil fuels generates 82 per cent of electricity in Massachusetts.
More than half of the life-cycle emissions from rail come not from the engines' exhausts, but infrastructure development, such as station building and track laying, and providing power to stations, lit parking lots and escalators.
"Any government considering expanding its rail network should take into account the emissions it will generate in doing so," Chester said.
"New rail systems should serve as links to other transit modes, as is often the case in Europe and Japan," he added.
"We should avoid building rail systems that are disconnected from major population areas and require car trips and parking to access," he explained.