Intermodal traffic moving in the United States during the third quarter grew by 4.7 percent year-over-year, according to data released yesterday by the Intermodal Association of North America (IANA). Across-the-board gains in intermodal traffic were outpaced, unsurprisingly, by domestic container volumes, the intermodal industry trade group's Q3 2013 Intermodal Market Trends & Statistics report said.

Domestic container traffic in the quarter rose 9.4 percent year-over-year, the tenth consecutive quarter it has grown faster than international container volumes, IANA said. It also marks the first time that seasonally adjusted domestic volumes exceeded those of international shipments, the group said. This stems from more than five years of increasing gains in domestic volumes and declines in international container volumes during the recession, followed by an uneven recovery, IANA said.

All told, domestic volumes rose 7.4 percent year-over-year. The total number included a 1.2-percent year-over-year gain in volumes of trailers, which has become a minor player in the domestic intermodal equipment landscape.

The Southeast led all regions in the quarter, reporting an 11.3 percent year-over-year increase, IANA said. That was followed by the Northeast, with an 8.3 percent year-over-year gain.

Domestic volume growth is being driven to some extent by the conversion of over-the-road truck shipments to the rail intermodal system. Traditional truck shippers are concerned about a possible shortage of drivers and equipment, increasing highway congestion, and the volatility of diesel fuel prices. As a result, they are turning to rail intermodal, which markets its operations as less costly, more fuel-efficient, and better for the environment. Service improvements by the railroads over the past decade have enabled intermodal service for shorter stage lengths, which were once dominated by truckers.

However, Joni Casey, IANA's president and CEO, said the group has no data to quantify the impact of truck-to-rail conversion on traffic trends.

For shippers and intermodal marketing companies, the fly in the ointment could be rising intermodal rates. These increases could drive traffic back to truck, where rates have remained surprisingly static despite the battery of trucker costs that, in theory, should be passed on in the form of higher prices.