It has become something of a truism that trains are the greenest mode of public transit (a corollary truism is that a human on a bicycle is the most energy-efficient mode of transportation, but that's another story entirely). But a new study published in the journal Environmental Research Letters (and reported in Green Inc.) calls that assumption into question.

The research, by Mikhail V. Chester and Arpad Horvath from U.C. Berkeley's Department of Civil and Environmental Engineering, is the first to measure not just the energy used per passenger-kilometer-traveled (PKT) during operations, but for a mode of transport's entire life-cycle.

For example, instead of just looking at how many miles per gallon your SUV gets, the researchers look at everything from the energy used in manufacturing your Expedition to the impacts of laying down all those roads to drive on.

Planes: not necessarily as bad as their reputation. Photo by hhsara.

The results bring perceptions of the climate impacts of mass transit down to earth. Perhaps unsurprisingly, automobiles are still the most impactful mode of transportation, but the real discovery is that the impacts of trains and planes are not as far apart as one would expect.

The (somewhat confusing) chart below compares the energy consumption and GHG emissions of 11 different methods of transportation per PKT, and finds that, for example, the San Francisco Muni metro light rail is more energy intensive than a Boeing 747, and Boston's Green Line light rail system is more GHG intensive than a Boeing 737 or 747.

The energy and GHG intensity of different transit methods. Click for full-sized.

The most important missing element of this study is long-distance rail travel, like Amtrak or the forthcoming high-speed rail in California, although the authors note that 83 percent of all rail travel in the U.S. is on short-distance systems like these.

The reasons for the proximity of impacts between rail and planes comes down to the infrastructure of the two modes; air travel is highly intensive in its operational use (aka when you're flying), but has minimal infrastructure. Train systems are much less intensive during operations, but require significant resources to build railways and train stations.

The Bay Area Rapid Transit (BART) system was one of the models for the study. Photo CC-licensed by ol slambert.

As a way of reducing the impacts of train travel, the authors write:

While energy efficiency improvements are still warranted coupled with lower fossil carbon fuels in electricity generation, reductions in station construction energy use and infrastructure operation could have notable effects. Particularly, the reduction in concrete use or switching to lower energy input and GHG-intensity materials would improve infrastructure construction performance while reduced electricity consumption and cleaner fuels for electricity generation would improve infrastructure operation.

Read the full report at IOP.org.