The AAR said Thursday 13,255 carloads of petroleum and petroleum products, or about 9.3 million barrels of oil, were delivered on the rail system for the week ending Feb. 1.
While slightly up from a year earlier, it was a 12.8 percent decline from the previous week.
John Gray, senior vice president for the AAR, said Thursday grain and petroleum products were the only commodity groups reporting increased shipments for last week.
Gray said it was likely that severe winter conditions affected shipments.
"We can't quantify it precisely but the extreme cold probably held down rail traffic to some extent," he said.
Since Jan. 1, the AAR said, 72,022 carloads, or about 50 million barrels of oil, were delivered on the U.S. rail system, a 10.4 percent increase from the same period last year.
Industry officials say the increase in U.S. crude oil production is outstripping the existing pipeline capacity, forcing some energy companies to turn to rail as an alternate transit option.
The AAR's railcar count excludes deliveries from the Canadian National Railway Co. and Canadian Pacific Railway.