High-speed rail is often the favoured answer but how will these “swish” trains measure up against the efficient cars and planes of the future? Is high-speed rail really the most environmentally sound long-distance transport option? A new study in Environmental Research Letters (ERL) weighs up the benefits and costs.

Do you need to go from Sacramento to San Diego? Right now the obvious way to make this 500 mile journey through California is to hop on a plane, but in the future a high-speed train might be an option too. To see if such a train really can help alleviate California's environmental woes, Mikhail Chester from Arizona State University and Arpad Horvath, from the University of California, Berkeley, carried out a life-cycle assessment framework of the different future-transport options.

Looking ahead to around 2050, the researchers compare a suite of possibilities, including more fuel-efficient cars and planes, electric cars, high-speed rail with clean electricity and high-speed rail with 100% renewable electricity. In addition to considering the direct emissions from each form of transport they also looked at the environmental impact associated with the infrastructure.

Currently Californians clock up 520 billion annual vehicle kilometres of travel (VKT) on their roads. By 2040 this is estimated to increase to 830 billion VKT, assuming no high-speed rail system is in place. Air travel is likely to accelerate at a similar pace too. "Without high-speed rail, we estimate that California will need an additional 1000 roadway lane kilometres, one to two additional runways, corresponding airport taxiways, and 32 additional airport gates," said Chester.

High-speed rail could reduce those requirements dramatically. "Existing studies estimate that high-speed rail will facilitate up to 33 million trips avoiding 5.8 billion automobile VKT and 5.1 million air trips annually," explained Chester.

In their model Chester and Horvath consider total energy used and atmospheric emissions (including greenhouse-gas emissions and air pollutants such as sulphur dioxide and particulates). They then consider a variety of scenarios, ranging from the present-day situation to a well-used high-speed rail system running on 100% renewable electricity.

The relative importance of infrastructure effects varied greatly depending on the type of electricity used. For high-speed rail running on traditional electricity mixes, the infrastructure effects can account for one-third to one-half of the total greenhouse-gas emissions. But if the electricity comes from 100% renewable sources, the infrastructure effects rise to around 95%, while total effects decrease.

In the case of California, Chester and Horvath's model showed that a state-of-the-art high-speed rail system would take around 20 to 30 years to pay back its greenhouse-gas emissions, assuming that the system is well used and reduces the number of road trips taken. "High-speed rail offers energy and environmental economies of scale when it is well utilized," explained Chester. So for California at least, high-speed rail seems to make sense.

Although the study was specific to California, Chester and Horvath believe that the findings also have broader implications. "The dominating life-cycle components that we highlight are likely to be the same dominating components for other high-speed rail systems," they said.