At least four people died and 13 others were injured after two bombs exploded close to Jaffer Express train that was travelling from Quetta to Rawalpindi in Bolan district's Machh area on Friday.
It is still not known whether the bomb was planted inside the train or on the tracks.
The injured have been shifted to hospitals in Quetta for treatment and authorities have declared emergency in all state-run hospitals.
No terrorist organisation has claimed responsibility for the blasts yet, Pakistani media reported.
Balochistan Chief Minister Nawab Sanaullah Khan Zehri has ordered an inquiry into the explosion while condemning the attack. He has also ordered for the immediate treatment of the injured.