The California Air Resources Board (ARB) has determined that locomotives contribute significantly to the air quality problems across the state. In 1987, locomotives accounted for 155 tons per day of oxides of nitrogen emissions (NOx). This contribution accounts for approximately 5 percent of the state’s total NOx emissions inventory. Currently, locomotives operating in California are not subject to any type of emissions mitigation program, except for some locally adopted opacity limits. Locomotives comprise one of the largest classes of uncontrolled NOx and oxides of sulfur (SOx) sources. Consequently, the ARB has determined that substantial NOx emissions reductions can be achieved by formulating and promulgating control strategies that target this source. For more information visit //www.arb.ca.gov/research/abstracts/92-930.htm.