Kamal Hyder, Al Jazeera's correspondent in Islamabad, said the immediate rescue operation was hampered because of darkness and fog in the area.
He said the number of dead was likely to rise as many of those injured are in a serious condition.
Asad Saeed, a railway official, said: "It appears to be an accident but we have ordered an investigation to ascertain the cause.
"Initial reports said a welded joint on the track broke, due to contraction in the extreme cold. It sometimes happens in winter."
|The train derailed near Mehrabpur, about |
400km north of Karachi
The carriages fell from an elevated part of the track about four metres above the ground, and piled atop each other in a twisted jumble of metal.
"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," a shaken Khan said.
"Somehow we managed to make it out."
The Karachi Express was filled with hundreds of people getting ready to celebrate the Muslim holiday of Eid al-Adha.
Officials said doctors and aid workers were struggling to cope with the large numbers of casualties.
Local people came by the hundreds to help, carrying the injured away on rickshaws, motor-scooters and donkey carts.
Hundreds of people have been killed in recent years on Pakistan's ageing railway system.
Casualty figures are often high because carriages are packed with many more people than they are designed for.