The improvised explosive device exploded as the train from Rawalpindi passed over it 180 kilometres east of Quetta, local security official Naseer Ullah told AFP.
"A passenger was killed and four others were wounded and moved to hospital," he said. A three-feet section of the track was destroyed by the bombing, which hit the second carriage.
Terror in Mastung: Bomb attack on train kills four
The injured were taken to hospital in the town of Sibbi, 25 kilometres north of the blast site. A provincial home ministry official in Quetta confirmed the details.
Rescue efforts were underway whereas police and security forces launched search operations to arrest the alleged perpetrators of the attack.
Meanwhile, Balochistan Chief Minister Sanaullah Khan Zehri while condemning the terror attack sought an inquiry report into the incident.
Further, the chief minister while expressing grief over the injuries inflicted to the people directed the local administration to provide best medical care to all the injured passengers.
This is not the first time that the Jaffar Express has come under attack. Most recently, in November 2015, four people were killed and another eight injured when the Rawalpindi-bound train hit a home-made bomb planted on a railway track in Mastung district.
UBA bomb attack on Jaffar Express in Sibi leaves 16 dead, 44 injured
An improvised explosive device (IED) was remotely detonated when Jaffar Express chugged into Dasht tehsil of Mastung, 30 kilometres south of Quetta.
In June 2015, the train was returning to Quetta when an explosion derailed four of its carriages near the Jacobabad district jail, leaving eight people injured.
No group immediately claimed responsibility for the attack but Baloch separatists demanding greater autonomy have been waging an insurgency for years and the province is also riven by sectarian strife and extremist violence.
The province's roughly seven million people have long complained they do not receive a fair share of its gas and mineral wealth.
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