The brakes apparently failed when the driver tried to stop to make way for a second passenger train that was leaving Khanewal, 250 miles south- east of Islamabad. The train was diverted on to a dead-end track, where it slammed into a pile of sand.
The first five coaches overturned, some rolling on top of others, making it difficult for rescue workers to reach those trapped inside. The 17- carriage train, carrying 1,500 people, was en route from Peshawar, in Pakistan's Northwest Frontier Province, to southern Karachi, on the Arabian Sea. Most of the dead and injured were asleep when the accident occurred.
The Prime Minister, Nawaz Sharifk, was flown by helicopter to the scene of the accident. He visited the hospital in Khanewal where hundreds of people had gathered to donate blood. He ordered an inquiry into the disaster, telling Khanewal's deputy commissioner to prepare an initial report within 24 hours.
Shahbaz Sharif, chief minister of Punjab, also visiting victims in hospital in Khanewal, said: "This is a horrific accident. It's a major accident in which 126 people have been killed."
People from Khanewal were the first to arrive on the scene. They pulled bodies from the overturned coaches and covered the dead in white shrouds. Many of the more seriously wounded were taken to Multan, 24 miles away. Hospital officials said that as many as 50 were in a critical condition and the death-toll could rise.
In Peshawar, the frontier provincial capital, hundreds of miles north of the accident, frantic relatives and friends were seeking information about those believed to be on the train.
Pakistan's train system is antiquated. In the past decade there have been dozens of accidents which have killed nearly 2,000 people. The most serious occurred in 1989, when two passenger trains collided in the southern Sind province, killing 850 people.