UP 9900 is the signature unit in a series of 25 locomotives, developed jointly with Electro Motive Diesel, that UP announced earlier it will use in a test of various emissions-reduction techniques throughout California.

To create the space needed to install aftertreatment technology in the body of the locomotive, engineers reduced the engine size of a long haul locomotive. The railroad expects UP 9900 to move toward the Tier 4 standard with a 45% reduction in the oxides of nitrogen emissions compared to the Tier 2 standard and an 85% reduction in particulate matter emissions based on preliminary analysis.

Union Pacific and the California Air Resources Board will jointly analyze the locomotive's emissions-reductions capability during the next 18 months.