Confirming the incident, Vinod Kumar, SHO, of government railway police, Sagauli in East Champaran, said 19 out of 21 persons on board the auto the auto rickshaw died on the spot.
The remaining two succumbed to their injuries at the sadar hospital at Motihari, the district headquarters town of East Champaran.
Officials said incident took place when the family members were returning to their native Chiknauta village under Sugauli police after performing pujaat a nearby temple.
The dead included the head of the family, identified as Subhash Pandey, 45.
Some eyewitnesses said although the crossing was manned, its gates were not closed when the mishap took place.
The incident occurred within hours after the Dehradun bound train had left the Muzaffarpur police station, its point of origin.
Top district officials had rush to the spot to take stock of the situation.