The mishap took place at a manned railway crossing at Chinauta village when Dehradun-bound Rapti Ganga Express hit the autorickshaw at full speed, Superintendent of Police Sudhir Kumar said.
Two other passengers suffered serious injuries in the mishap.
Some bodies were mutilated beyond recognition while some others were dragged by the train to some distance as the vehicle was tossed several metres away under the impact of the collision.
The dead and the injured all belonged to a single family from Chiknauta village, the SP said adding the injured persons had been admitted to a government hospital in Motihari where they were battling for life.
Meanwhile, the ECR Samastipur Division's Divisional Railway Manager Arun Kumar Malik has left for the spot by a special train to supervise rescue and relief work.
The Superintendent of Railway Police Muzaffarpur Vinod Kumar has also rushed to Motihari to assist in the relief work.
In Patna, Bihar Chief Minister Jitan Ram Manjhi has expressed heart-felt grief over the loss of lives in the accident and announced Rs 1.5 lakh compensation to the kin of each victim.
A report from Motihari said that the train service on Sugauli-Semra section has been disrupted after the mishap.
Efforts were on to remove the mangled remains of the autorickshaw from the railway track to restart train services on the route, they said.
A crowd of villagers gathered on the spot and raised slogans demanding Rs 2 lakh compensation for each of the victim. The SP intervened to calm the agitators.
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