Intermodal traffic in May was up from a year ago, thanks to growing international trade and other factors.

The Association of American Railroads reported that intermodal traffic in May increased 7.5 percent for a total of 932,956 trailers and containers compared with May 2010. On a seasonally adjusted basis, carloads were flat and intermodal was up 0.8 percent over April 2011.

May saw the 18th straight month of intermodal gains, and this month's weekly average of 233,239 is the second highest May average on record. The gains in intermodal can be attributed to several factors, says AAR, including growing international trade, better service, large investments in infrastructure and equipment by railroad companies, fuel costs, highway congestion and truck driver shortages, and the conversion of boxcar traffic. Looking further at the import and export commodities, "big box" retailers dominate intermodal container imports while recycled paper, scrap materials, and chemicals dominate container exports.

"For the second month in a row, rail intermodal traffic was great, while carload traffic left something to be desired," said AAR Senior Vice President John T. Gray. "Like other national indicators, rail traffic reflects a degree of uncertainty regarding the direction of the economy. Railroads join everyone else in hoping current trends are just a bump in the road rather than a portent of things to come."