"The explosions damaged two passenger carriages and killed six people besides wounding 19 others," Imtiaz Ahmad, a senior local Pakistan Railways official, told AFP news agency, updating an earlier toll of three deaths.
The attack was claimed by the separatist Baloch Liberation Army, according to AFP.
A spokesman for the group said the bombing had targeted "military personnel who travel to Rawalpindi by this train".
Kashif Akhtar, a senior railways official, told Reuters that security forces foiled a similar attack on Thursday as an attacker laid explosives on a railway track near Quetta.
Railways Minister Khawaja Saad Rafique denounced the bombing as "an act of terrorism" and said authorities were still trying to determine how the bomb was planted on the train.
For more than a decade, Balochistan has been the scene of low-intensity attacks by separatist Baloch groups who want autonomy or outright independence.
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