The IEA said it reviewed information from the Department of Transportation and determined rail deliveries were six times more likely to result in a spill than pipelines from 2004-12.
"Increasing volumes of crude oil transported by rail raise questions of safety," the IEA stated in a Bloomberg News report. "Our analysis reveals that compared to pipelines, rail incident rates are higher while the opposite holds for spill rates."
Bloomberg didn't report on specific spill volumes, however. An April train derailment in Minnesota spilled around 350 barrels of oil. A March pipeline rupture in Arkansas released about 5,000 barrels of oil.
The Association of American Railroads said the weekly average for trailers and containers last month was around 240,000, the highest average for weeks in April.
The U.S. Energy Department said rail shipment of petroleum and petroleum products is more expensive than pipelines but generally offers energy companies a greater degree of flexibility in terms of destination points.
Energy companies are turning to rail to deliver oil because the production boom in the United States has outpaced existing pipeline capacity. AAR information on spills was in contrast to the IEA's report.