A diesel locomotive travels in Union Pacific's Council Bluffs, Iowa, rail yard, Tuesday, Jan. 7, 2014. The boom in natural gas production that dramatically changed the nation’s energy outlook may now be on the verge of shaking up American railroads. Rail companies are considering replacing diesel locomotives, which have been the workhorse of freight railroads since World War II, with redesigned engines that could burn natural gas as well. They believe the new models could cut fuel costs by up to 50 percent and reduce pollution, potentially changing the economics of large-scale shipping. (AP Photo/Nati Harnik)
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