The SD59MX locomotive, which was rebuilt from an Electro-Motive Diesel (EMD) SD60M, is being used on workings with a range of up to 320km from its base at UP's J.R. Davis yard in Roseville, California.

Union Pacific engineers worked closely with EMD to reduce the size of the engine in order to accommodate EGR, DOC, and DPF. The 2.24MW locomotive, numbered UP 9900, uses all three aftertreatment technologies simultaneously, and will help UP to develop locomotives that meets the US Environmental Protection Agency's Tier 4 standard. Indeed, UP and EMD expect the SD59MX to move towards Tier 4 with a 45% reduction in nitrogen oxide emissions compared with the Tier 2 standard and an 85% reduction in particulate matter emissions based on preliminary analysis.

UP and the California Air Resources Board will jointly analyse the locomotive's emissions reduction capability over the next 18 months.

UP 9900 is the most advanced of 25 units being used by UP to test a variety of emission-reduction systems in northern and southern California.