Unknown militants fired a rocket-propelled grenade at Jaffer Express near Bakhtiarabad area of Sibbi district, when it was on its way to the provincial capital Quetta.
A police official said that one passenger was killed and five other injured in the attack.
"One of the carriages was also slightly damaged," he said.
Paramilitary forces and police arrived at the place of attack and launched search operation.
Attacks on trains are common Balochistan where Baloch nationalists are fighting for greater control over natural resources.
So far no groups claimed responsibility.
Balochistan Chief Minister Sanaullah Khan Zehri condemned the terror attack and sought an inquiry report.