The hybrid locomotive, based in Fresno, is powered by storage batteries. A diesel engine recharges the batteries while the locomotive isn't being operated.
Anthony Presto of the San Joaquin Valley Air Pollution Control District says the hybrid engine emits 80 to 90-percent fewer emissions than a diesel-fueled locomotive.
"That will reduce nitrogen oxide emissions by 132 tons over the life of the engine."
Spokesman John Bromley says Union Pacific is considering adding more of the high-tech engines to it's fleet, in the highly polluted valley region.
"Ten more of those are going to go down to Texas in the next couple of months, and then we're looking back at California for some potential additions."
Union Pacific hasn't determined the total cost of developing the hybrid locomotive, but Bromley says California public agencies, including the regional Control District, have contributed 700-thousand dollars to the project.