A powerful bomb on a railway track derailed a passenger train near Bahawalpur Friday, killing at least four people, including three children, an official said. The explosion badly damaged two carriages of the train and also injured at least 15 people, senior railways official Khalid Amin said. "A bomb was planted on the track which exploded when the train reached there," he told a foreign news agency. He initially gave a toll of two dead but said two critically injured men died later in hospital. A woman and three children were the others who were killed, he said, adding that an investigation had been ordered into the incident. There was no immediate claim of responsibility.