The Environmental Protection Agency (EPA) is finalizing a comprehensive three-part program to reduce emissions of particulate matter (PM) and oxides of nitrogen (NOx) from locomotives and marine diesel engines below 30 liters per cylinder displacement. Locomotives and marine diesel engines designed to these more stringent standards will achieve PM reductions of 90 percent and NOx reductions of 80 percent, compared to engines meeting the current Tier 2 standards. These standards will also yield sizeable reductions in emissions of nonmethane hydrocarbons (NMHC), carbon monoxide (CO), and hazardous compounds known as air toxics. This program is part of EPAs ongoing National Clean Diesel Campaign (NCDC) to reduce harmful emissions from diesel engines of all types. The anticipated emission reductions will significantly reduce exposure to harmful pollutants and also provide assistance to states and regions facing ozone and particulate air quality problems that are causing a range of adverse health effects, especially in terms of respiratory impairment and related illnesses. We project that by 2030, this program will reduce annual emissions of NOx and PM by 800,000 and 27,000 tons, respectively. The annual monetized PM2.5- and ozone-related health benefits of this rule in 2030 will range from $9.2 billion to $11 billion, assuming a 3 percent discount rate, or between $8.4 billion to $10 billion, assuming a 7% discount rate. The estimated annual social cost of the program in 2030 is projected to be significantly less, at $740 million. This Regulatory Impact Analysis provides technical, economic, and environmental analyses of the emission standards. Chapter 1 provides industry characterization for both the locomotive and marine industry. Chapter 2 presents air quality modeling results and describes the health and welfare effects associated with particulate matter (PM), ozone, and air toxics. Chapter 3 provides our estimates of the current emission inventories and the reductions that can be expected from implementation of the more stringent standards. Chapter 4 contains our technical feasibility justification for the emission limits, and Chapter 5 contains the estimated costs of complying with those standards. Chapter 6 presents the estimated societal benefits of the rulemaking. Chapter 7 contains our estimates of the market impacts of the more stringent standards and the distribution of costs among stakeholders. Finally, Chapter 8 contains our analysis of several alternative control scenarios we considered during the development of this rulemaking.