The accident occurred soon after the train left the Dulhazara station in Chakaria Upazila
Nine others have been injured.
Sayestaganj Highway Police OC Ezazul Islam said a truck rammed into an auto-rickshaw from behind around 7.30am amid dense fog on the highway.
"The impact hurled the auto-rickshaw against a four-wheeler passenger vehicle and a tractor, causing the casualties," he said.
Four among the dead have been identified as Noubul Bibi, 50, her daughter Ambia Khatun, 30, Babul Mia, 40 and Ahamdul Haque, 45, according to OC Islam.