The group was crossing the tracks after performing Chhath Puja and did not see the approaching train due to morning fog.
The accident took place near Rambhadrapur railway station, around 35 km from Samastipur town, Senior DCM Samastipur division, Birendra Kumar said.
While three persons died on the spot, one succumbed to injuries at the hospital, he said.
The injured have been sent to Darbhanga Medical College and Hospital (DMCH).
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