Eight train wagons fell from the track which was going from the troubled northwestern city of Peshawar to Karachi, he added.
Railways police official Raja Zaheer Arshad also confirmed the blast and casualties.
Nobody immediately claimed the responsibility for the bombing, he added.
Since July 2007, Pakistan has been gripped by a local Taliban-led insurgency, concentrated largely in the northwest.
The government says more than 40,000 people have been killed in violence since 2001, when Pakistan allied itself with the US-led "war on terror".