The traders were returning to their neighbourhood after buying vegetables from the wholesale market in Manikganj’s Jaagir.
The accident took place in the Bou Bazar area of Nilphamari Sadar Upazila at 8 am on Wednesday, according to Nilphamari Sadar Police Station chief Abdur Rouf.
The dead have been identified as three siblings - Rima Akhter, 7, Reshma, 4, Mominur Rahman, 3, - and Shamim Hossain, 26.
The three siblings were playing on the rail tracks near their home when a Chilahati-bound train from Khulna ran them over, killing them on the spot, locals said.
Shamim attempted to save the children and was severely injured. He was declared dead after he was taken to a hospital, said police official Abdur Rouf.