AAR said the commodities with the largest increase for August were petroleum and petroleum products, which increased 18.5 percent year-on-year. AAR said 8,148 carloads, or approximately 5 million barrels of oil, traveled by rail in the United States last month.
AAR has posted steady gains in the amount of crude oil delivered by rail in the United States this year.
AAR Senior Vice President John Gray said in a statement Thursday "it wouldn't be surprising" to see records set this month and in October because fall is typically the peak season for rail traffic.
Oil production in the United States has increased to the point that existing pipeline capacity isn't enough to handle the additional volume.
U.S. regulators last week announced they were reviewing rail safety in the wake of an oil rail disaster in Lac-Megantic, Quebec. At least 40 people were killed when a train carrying tankers of crude oil from North Dakota to Canadian refineries derailed there in July.
AAR said coal delivery by rail was down 2 percent and grain was down 9 percent from August 2012.