Pakistan Railways ruled out sabotage and said that a piece of track appeared to have broken in the extreme cold, sending the 17-carriage train hurtling over an embankment outside Mehrabpur railway station. Several carriages were crushed or torn to shreds, they said.
"We hope there are no bodies left inside," said Brigadier Mazhar Jamil, who led the army rescue team that worked through the night and morning, treating the wounded and picking through the mangled wreckage for the dead.
The train was en route to Lahore, filled with hundreds of people getting ready to celebrate Eid al-Adha, when the train derailed outside Mehrabpur railway station, 200 kilometres (125 miles) north-east of Karachi.
The carriages then slid down an embankment four metres high. Some simply tipped over while others were ripped open, as terrified survivors scrambled through the pitch dark and freezing cold to safety.
The Operations Manager, Asad Saeed, told AFP that 45 bodies had been counted and that more than 100 people had been injured. But he later acknowledged that there had been differing tolls throughout the course of the day - from between 28 to 56 dead. Trains are often overcrowded and it can be difficult to know how many passengers are aboard.
Brigadier Jamil said that countless lives were saved by local residents who fought off the cold and hauled the wounded away in the darkness before rescue teams arrived, taking them to hospital on rickshaws, scooters and donkey carts.
But for many survivors, there was tragic news. Rescue teams found a badly injured four-year-old girl, weeping in pain amid the debris. Her parents were missing, and presumed dead, said Abdul Hameed of the Edhi Welfare Trust, a major national aid group.
"We have tried to find her relatives," Hameed told AFP. "But nobody has come forward." Several carriages were destroyed, and wheels from the train were found a half-kilometre from the site of the wreck. Rescue workers gathered the scattered belongings of passengers that littered the site --shoes and quilts, schoolbooks, broken furniture--in a separate tent, waiting for those owners who were still alive to come and claim them.
"We were almost asleep when we heard something--a big bang. Then I felt I was flying through the air and the carriage was tumbling to the ground," said Shahid Khan, a 24-year-old salesman. "We were grappling in the darkness. Somehow we managed to make it out," a shaken Shahid said.
"People were screaming and scrambling to get out," said Mohammad Jamil, who fractured his arm. "It was the middle of the night, and we couldn't see anything."
He said that the military found no signs of foul play and Saeed, of Pakistan Railways, said 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 Wednesday's crash.