Total intermodal volume rose to 201,188 units, from 191,511 last year, AAR said in its weekly report.
Containers rose 8.3% to 170,514 units, while trailers fell 10.1% to 30,674 units.
Freight rail traffic, which excludes intermodal traffic, grew 1.4% to 268,033 carloads, AAR said.
Railroad volume is considered an important economic indicator. Intermodal traffic, which tends to be higher-valued merchandise than bulk commodities, uses trains for the long haul and trucks for the shorter distance at either end of the trip.