By Colin Kellaher

North American rail traffic fell 2.3% last week, as declining carloads once again more than offset a rise intermodal volume, data from the Association of American Railroads showed.

Carload volume fell 9% for the week ended Sept. 26 on 12 reporting U.S., Canadian and Mexican railroads, while intermodal traffic rose 4%, the trade group said Wednesday.

In the week ended Sept. 19, North American rail traffic fell 1.4%. For the first 39 weeks of the year, North American traffic is now down 10%, compared with a year-to-date decline of 10.2% reported a week earlier.

The AAR said U.S. rail traffic fell 1.3% last week, with carloads down 10.5% and the volume of intermodal containers and trailers up 5.5%. U.S. rail traffic is now down 10.7% for the year to date, the AAR said, compared with a decline of 10.9% reported a week ago.

Canadian rail traffic was flat last week, as a 3.8% rise in intermodal units offset a 3.4% decline in carloads. Canadian rail traffic is down 7.7% for the first 39 weeks of the year.

Mexican rail traffic slid 13.5% for the week, as carloads tumbled 12.9% and intermodal units slumped 14.2%. Mexican rail traffic is down 9.8% for the year so far, the AAR said.

Write to Colin Kellaher at colin.kellaher@wsj.com