Nearly 40 children of a private school, mostly below 11, were in the bus when the accident took place in the Medak district, 70 kms from the state capital of Hyderabad. After the bus was hit by the train, witnesses say it was dragged about a kilometre down the tracks.
The police say 21 children have been injured and hospitalized in Secunderabad 60 km away. The bus cleaner and the father of one of the children, who collapsed on reaching the site, also died.
As rescue workers tried to pull out the children, parents sat with bodies in their laps, many mothers holding the children they had prepared for school just an hour before. One mother was seen kissing her child repeatedly.
Schoolbags and water-bottles lay scattered under the mangled remains of the bus as hundreds of people sat and mourned, not all related to the children.
The accident sparked anger among local people, who say they have been asking for gates to secure the railway crossing in the district. In Parliament, Railways Minister Sadananda Gowda faced loud protests as members demanded to know why safety was neglected.
About 15,000 people are killed each year while crossing the tracks on India's vast but decrepit railway network, according to a government safety panel. Two years ago, the panel recommended that more bridges and overpasses be built on priority.
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