The Khushhal Khan express train from Karachi and bound for Peshawar in the northwest derailed in the southern town of Jacobabad, Geo News reported.
The report said the explosives planted in the tracks were detonated as the train passed over them.
China's Xinhua news agency, quoting other local media and railroad officials, said at least 20 more people were injured in the incident just after midnight. The report said two cars of the train jumped the damaged tracks.
Rescue work was continuing, the report said. Jacobabad is in Sindh province of which the port city of Karachi is the capital.
It was not known if any individual or group had claimed responsibility for the attack, but the region has been the scene of much sectarian or militant violence for months.