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| Fig. 1: Beijing-Shanghai High Speed Rail (Source: Wikimedia Commons). |
Rails have a checkered past when it comes to energy and the climate. Its long history and association with steam engines and the industrial revolution do not seem to fit the vision of a greener future. However, from metros and light rails in major cities, to high-speed trails connecting urban centers, in recent years rails of all types have featured more and more prominently in plans for future transportation. To better understand this trend and the role of rails in a climate-conscious future, in this report we examine the numbers behind the energy demand and environmental footprint of modern rail systems.
In 2019, the International Energy Agency published a report based on input and data from member states and the International Union of Railways (UIC). [1] In the report, they classified modern rail systems into several categories. Based on the service they provide, rail systems can be broadly separated into passenger and freight rails. Among passenger rails urban and high-speed rails are further distinguished from conventional trains. Specifically, urban rails include metro and light rails, while high speed rails refer to rail services operating at a maximum speed above 250 km per hour. In 2016, rail accounted for around 8% of total transport passenger-kilometers, for a total of over 4 trillion kilometers traveled. 90% of rail passenger activity is concentrated in a few regions: China, Europe, India, Japan, and Russia. In contrast, rail networks in North America mainly serve freight transport. For passenger transport, high-speed rails in particular have seen a rapid expansion in recent years. By the time of the report, 600 billion passenger-kilometers are traveled by high-speed rails annually, compared to 3100 billion by conventional rail. The biggest growth comes from China, which now accounts for two-thirds of global high-speed rail tracks, virtually all built within the last decade. With rails already large and ever-growing footprint, it is important to examine the energy and environmental implications of modern rail systems.
According to the IEA report, on a global basis railways today consume around 2% of total transport energy use, a small share compared to cars and aviation. [1] This is especially significant compared to rails larger share of transport activity: 8% of total passenger-kilometers and 7% of total tonne-kilometers. This higher energy efficiency is attributed to several factors. Rail in general benefits from larger capacities, carrying more passengers and goods per vehicle. It also benefits from infrequent stopping, with dedicated traffic and right-of-way. In addition, compared to cars, the rolling friction energy loss of steel-to-steel contact of rails is far lower than those of rubber tires (e.g. order of ~90% lower than truck tires). Finally, rail systems today rely on electric motors as opposed to less efficient engines to a much greater extent than other modes of transport. Globally, almost half of rail energy consumption is in electricity. In major economies like China and Europe, the share is 70%, and reaches more than 90% in the case of Japan. Using historical data available up to 2016, the IEA developed the IEA Mobility Model to make quantitative projections of transport activity and energy demand. [1] By their model, if all services currently performed by railways were carried by road vehicles, then the world's transport-related oil consumption would be 8 million barrels per day or 15% higher. By these metrics, a future with more rail transportations is certainly one aligned with less fuel consumption and higher proportion of clean energy like electricity.
The global statistics presented in the last section gives a good idea of the overall energy demand and efficiency of modern rail systems. In this section, we examine specific case studies including the proposed Europabanan line in Sweden, and the Beijing-Shanghai high-speed line in China (Fig. 1), to better understand the environmental impact in the lifecyle of railway systems. In both cases, a life-cycle perspective is crucial for assessing the overal impact of rail travel. The operation of trains is not the only source of emission coming from rail transport. It also arises from the construction and maintenance of the rail network. For the Swedish study, the Europabanan is a proposed 300 km/h high-speed line from Stockholm to Gothenburg. [2] Beyond passenger travel, the line is also projected to increase the capacity for freight trains by a factor of 2 to 3. The authors examined the green gas emission impact of the proposed line, taking into account the construction and operation of the rail network, as well as indirect effects to fuel demand, vehicle use and manufacturing, and air travel operations. As a result of these factors, the authors estimated 55,000 tons of CO2-equivalent emission reduction per annum from the new rail line. Interestingly, while 40% of that reduction comes from the passenger side with the shift from air and road travel to high-speed rails, the shift from truck to freight trail transport accounts for 60% of the reduction. This highlights the advantage of a multi-puporse approach to rail development, and the important part freight rail can play in addition to the much more talked about high-speed passenger transport.
In the Beijing-Shanghai line study, researchers examined the carbon footprint of the most trafficked line in China connecting two of the biggest Chinese cities, using a similar life-cycle perspective. [3] Using data from 2011 to 2014, they estimated that operation of the line accounted for ~70% of its CO2-equivalent emissions, while the construction stage accounted for ~20%, with maintenance 10% of total emissions. In that period, the Beijing-Shanghai high speed has a per-passenger carbon footprint of around 50 grams of carbon per kilometer. This is significantly higher than other high speed lines around the world compared in this study. For example, the carbon footprint of the Tokaido Shinkansen line in Japan is only around 5 grams of carbon per kilometer. This larger carbon footprint is mainly attributed to China's dominant use of coal for electric generation. To unlock high-speed rails full emission reducing potential in China, the author suggested that cleaner electricity supply options and more efficient raw material production are needed in the future.
Modern rail systems, in all its diverse forms serving both passenger and freight transport, are highly electrified and more energy efficient than most other modes of transport. However, currently rail only accounts for 2% of total transport energy use. Thus, simply building more rails is unlikely to have a significant impact on the overall trend of energy demand and carbon emissions. In some cases, economic incentives will still favor other modes of travel despite being less energy efficient, such as airplanes over high- speed rails for long-distance travel, and cars over metro systems in cities. However, in many other areas where incentives do align rail-systems have been shown to play a promising role in reducing energy demand. For example, a study of rail development and air patronage in China from 1993-2012 shows that railway development is associated with a reduction in air transport for short- and medium- haul intra-city travels, although they did not find a significant association for long-distance hauls of over 1000 km. [4] This highlights the importance for future rail development to take into these economic incentives when considering which area of rail development to focus on.
Lessons from individual cases of modern rail projects also suggest that simply building more rails is not the solution. It takes a balanced approach that closely examines the emission impact across all stages during the life cycle of a rail system. Furthermore, comparison of the energy efficiency of high-speed rail systems in different countries illustrate that rail is only part of a larger energy eco-system, and can only be as energy efficient and environmentally friendly as the dominant means of energy and material production in that system.
© Minjie Lei. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "The Future of Rail, Opportunities For Energy and the Environment," International Energy Agency, January 2019.
[2] J. Lin et al., "A Carbon Footprint of High-Speed Railways in China: A Case Study of the Beijing-Shanghai Line," J. Ind. Ecol. 23, 869 (2019).
[3] J. Åkerman, "The Role of High-Speed Rail in Mitigating Climate Change - The Swedish Case Europabanan From a Life Cycle Perspective", Transp. Res. D. Transp. Environ. 16, 208 (2011).
[4] L. Li and B. P. Y. Loo, "Railway Development and Air Patronage in China, 1993 2012: Implications for Low-Carbon Transport," J. Reg. Sci. 57, 507 (2017).
