An abundance of cheap natural gas from new domestic production sites is dampening demand for coal, eroding what has traditionally been a core source of revenue for U.S. railroads. The number of carloads of coal hauled by U.S. railroads during the first 29 weeks of the year through July 21 is down 10 percent from a year ago to 3.3 million, according to the Association of American Railroads (AAR). Other commodities with negative growth this year include grains, iron and s...