About 71 people were aboard the train at the time, northjersey.com reported.
The driver of the truck was not injured. He was trying to make a tight turn onto a street in Little Falls while the crossing gates were up, but backed up when he couldn't complete the turn, said John Durso, a spokesman for New Jersey Transit.
As the driver was reversing the tractor-trailer, the crossing gates came down and the train hit the trailer, Durso said.
The train came to a stop 400 feet down the track.
The truck was carrying 5-gallon containers of paint, which was splattered over the road and track by the crash.
Officials did not know how much of a factor the fog played in the accident.