The Association of American Railroads estimates that around 241,000 rail tankers hauled crude oil and petroleum production during the first six months of 2012, a 38 percent increase from the previous year.
The U.S. Department of Energy's Energy Information Administration attributes the increase in rail deliveries to expanded production in the Bakken oil formation in North Dakota where pipeline options are limited.
Oil production in North Dakota has increased each year for the past four years. The U.S. Geological Survey in 2008 estimated there were 3 billion-4.3 billion barrels of oil in the U.S. part of the formation.
The International Energy Agency notes that while the transportation sector is responsible for nearly 25 percent of global energy-based carbon dioxide emissions, railway companies account for less than 1 percent of the total.
Efficiency, meanwhile, is improving for rail. The IEA said, in a report, that railways used 13 percent less energy in 2009 than in previous years.