Railroads ship massive amounts of sand to North Dakota and other states where hydraulic fracturing, or "fracking," is used to recover oil and gas.•Each horizontal well in North Dakota requires between 3,000 and 10,000 tons of sand.
•So far, just under 4,000 wells have been drilled in western North Dakota. The state's Oil and Gas Division estimates that another 36,000 wells could be drilled.
•Shipments of sand and gravel over major U.S. railroads grew 7 percent in 2011.
Source: North Dakota Oil and Gas Division, Association of American Railroads, Union Pacific Railroad
