The bomb, hidden in a tea stall on the platform of Sibi railway station, was triggered by remote control late tonight while the Jaffar Express train was at the station.
“Five persons, including a railway policeman, were killed and 18 were injured. Eight of the injured are in a critical condition,” Deputy Commissioner Shahid Saleem Qureshi told the media.
Preliminary investigations had shown that the bomb was triggered by remote control, Qureshi said.
Security forces cordoned off the area after the blast.
The Jaffar Express train was going from Rawalpindi to Quetta.
No group claimed responsibility for the blast though such attacks are usually blamed on Baloch nationalist groups.