The accident took place at the unmanned Rajghat railway crossing near Bettiah, about 200 km from Patna, when the car carrying people from a wedding including the groom, was hit by the goods train.
"Four people were killed on the spot and seven succumbed to their injuries at a hospital," a district police official said.
The three injured, including the groom, are battling for their lives.
There were 14 people in the car.
According to police, all the victims were from one family.