The United States is poised to export the most coal since 1992 this year, filling a gap created when flooding interrupted Australian supplies and buoying shipments for railroads such as Union Pacific and CSX. Union Pacific, the nation’s largest railroad, may double coal exports this year to more than 4 million tonnes, said Doug Glass, the company’s vice president and general manager for energy. CSX predicts shipments may rise 33 percent to a record in 2011 after first-quarter volumes climbed 45 percent.