The Association of American Railroads' (AAR) recent move to oppose any legislation allowing longer twin-trailers to operate on the National Highway System was not influenced by its core membership of Class I railroads but by short-line and regional carriers that have fought changes to the 36-year-old law, an advocacy group supporting longer combination vehicles said today.

The 28-member group, "Americans for Modern Transportation," said in a statement that the AAR's abrupt about-face from what had been a largely indifferent stance on the issue came "on behalf of the nation's short-line railroads" which the group said is "holding productivity hostage as a means of holding back their competition" in the face of potential improvements in highway efficiency that the longer trailers would provide. The American Short Line and Regional Railroad Association (ASLRRA) did not reply to an e-mail request for comment. An AAR spokeswoman declined comment.

Sources close to the Americans for Modern Transportation group said their members have been told individually by representatives of large or "Class I" railroads that they remain neutral on changes in trailer size because it is of peripheral concern to them. Some of the big railroads said they were uncomfortable with the AAR's policy shift because they didn't want to alienate some of the advocacy's group members who are also large rail users, according to these sources.

The coalition includes major ground parcel and less-than-truckload (LTL) carriers, various shipper groups, e-tailer Amazon.com Inc., the National Retail Federation, the Retail Industry Leaders Association (RILA), the U.S. Chamber of Commerce, and food supplier giant Sysco Corp. The group is lobbying to include language calling for longer trailer sizes in the fiscal year 2019 appropriations bill to fund the Department of Transportation.

Most railroads, as well as the AAR until recently, have been focused on blocking any legislative efforts to lift the 80,000-pound limit on a truck's gross vehicle weight, which is the combined weight of tractor, trailer, and cargo. That's because higher truck weights pose a more direct threat to the rails' competitive position. However, in its recent policy change AAR said that increases in both truck size and weight limits would damage the country's infrastructure. In the past, AAR reserved that argument to oppose any changes in truck weight.

Since 1982, federal law has capped at 28 feet the length of each trailer on a twin-trailer hook-up. Shipper groups, parcel, and less-than-truckload carriers have fought unsuccessfully to get legislation passed to extend each trailer to 33 feet. They argue that longer trailers would boost truck efficiency and productivity by adding 16 percent of cubic capacity to each run without any increase in weight that could damage the country's infrastructure.

The measure would be critical in meeting the expected increase in demand for freight hauling, especially as e-commerce means more goods are to be delivered, supporters have said.

Opponents, which have included safety advocates, independent owner-operator drivers, and most major truckload carriers, have said that extended trailers pose a safety risk because the highway system's merge lanes and on-off ramps were not designed to accommodate tractors carrying twin-trailers longer than 28 feet each. Supporters have countered that the longer trailers are equipped with extended wheelbases that allow for more stability than what is available to rigs hauling 28-foot equipment.

AAR has backed the short lines before on legislative and regulatory matters, such as the extension of tax credits for short-line rail operations. The seven Class I railroads that operate in the U.S., including the U.S. operations of Canadian National Inc. and Canadian Pacific Railway, comprise AAR's core membership.