This paper reports some results from a study of emissions control for railway locomotives performed for the California Air Resources Board.(1)* Feasible and cost-effective control techniques for locomotive emissions include retarding injection timing and other engine modifications, selective catalytic reduction (SCR), use of liquified natural gas (LNG) fuel with low-emission dual-fuel or spark-ignition (SI) natural gas engines, LNG combined with SCR, and electrification. Use of a combination of dual-fuel and SI LNG engines could reduce locomotive NOx emissions by 80%, at a cost of less than $1,100 per ton of NOx eliminated. SCR added to diesel and LNG could produce NOx emission reductions of 90 and 97 %, respectively, at costs less than $3,300 per ton. All of these technologies could be retrofit to existing diesel locomotives. Electrification of line-haul locomotive operations would give an even greater NOx reduction, but the cost would be ten times that of the next most expensive option, and the incremental NOx reduction over the best non-electric approach would be small.Railway locomotives constitute one of the largest remaining uncontrolled NOx sources in many areas, including California. The objectives of the study reported here were: 1.to identify a set of feasible and cost-effective techniques to reduce locomotive emissions in California to the greatest extent possible at an acceptable cost; 2.to characterize the technical requirements, costs, emission impacts, and impact on railway operations of each technique in sufficient detail to serve as a basis for regulation; 3.to identify and recommend areas where ARB or other public funding for additional research, development, and demonstration of specific techniques are required in order to make them available for widespread application; and 4.to develop and recommend a regulatory strategy and implementation schedule for reducing locomotive emissions in California as quickly and cost-effectively as possible, and estimate the emission benefits which would result. This paper presents some of the main results and conclusions of the study. More detailed information is given in the final report to the Air Resources Board (1).