Comparison of emissions from light rail transit, electric commuter rail, and diesel multiple units

The United States is experiencing a revival of interest in diesel multiple-unit (DMU) passenger railcars. At the same time, communities are questioning whether diesel makes good sense as they compare the diesel multiple unit with electric rail modes such as light rail or electric multiple units. This study quantifies the emissions into a region from diesel multiple units and the electricity generation for electric rail modes, for oxides of nitrogen (NOx), an ozone precursor; particulate matter (PM); volatile organic compounds (VOCs), another ozone precursor; carbon monoxide (CO); and carbon dioxide (CO2). The study found that emissions attributable to electric rail modes are highly variable and depend on the cleanliness of the electricity generated. The dirtiest electricity pollutes orders of magnitude more than the cleanest. The study conclusion is that emissions from diesel multiple units and electric rail modes are not dramatically different on a per seat mile basis and that the exact comparison will depend on the cleanliness of the electricity generation and the type of diesel multiple unit consist. When compared on a per seat mile basis against electric rail modes using the average electricity generated in the United States, DMUs emit about the same amount of PM, equal or greater amounts of NOx, more CO and VOCs, and less CO2. The study also concludes that the rapidly changing diesel engine emissions standards will result in DMUs being introduced within the next 6 years with PM and NO, emissions that are nearly as low as the cleanest electricity generation today.