Pakistan Railways ruled out sabotage and said a piece of track appeared to have broken in the extreme cold, sending the 17-carriage train hurtling over an embankment outside Mehrabpur. Several carriages were crushed or torn to shreds.
The Karachi Express was en route to Lahore, filled with hundreds of people getting ready to celebrate the Muslim holiday of Eid al-Adha, when the train derailed outside Mehrabpur, 200 kilometres (125 miles) northeast of Karachi.
The carriages then slid down an embankment four metres high. Several of the train carriages were completely destroyed.
Operations manager of the state-run rail company, Pakistan Railways, Asad Saeed told news agency AFP that there was apparently a metal failure on the track. "Initial reports said a welded joint on the track broke, due to contraction in the extreme cold," he said. "It sometimes happens in winter."
A relief train was sent to take survivors away from the scene of the crash.