The use of exhaust aftertreatment technologies may be required to meet the future Tier IV locomotive emissions standards that have been proposed by the US-EPA. Diesel Particulate Filters (DPF) can achieve a high level of particulate matter (PM) emissions reduction, and they are currently being used on all new 2007 on-highway diesel trucks in the United States. In addition, DPFs have been installed on lower power (1500 kW and less) locomotives in Switzerland that are fitted with 4-cycle high speed diesel engines. However, these systems are only now beginning to be sized and demonstrated in a locomotive environment in the United States. Initial testing of the retrofitted DPF system on a 1,125 kW two-stroke, EMD 12-645 Roots-Blown engine powered locomotive achieved an average 80% reduction in Particulate Matter and an average of 30% reduction in Hydrocarbon from the baseline emissions levels. This paper will discuss the background of the “California Emissions Program”, the current and recently proposed EPA emissions standards for locomotives, the Diesel Particulate Filter (DPF) used in the field demonstration, and emission test results.