The car jumped the road divider, went into a lane on the other side and collided head-on with a truck coming from the opposite direction, the police said.
A five-member high-level committee has been constituted to investigate the incident.
The constable fired from his automatic weapon, killing an RPF Assistant Sub Inspector and three other passengers of the Jaipur-Mumbai Central Express shortly after 5 am, the official said.
A video of family members with the child in their arms crossing a river to reach the PHC has gone viral on social media.
Two persons were swept away in swollen water bodies in the last two days. While the body of one of them was recovered, efforts were on to trace the other person, the police said.