An express train crowded with holiday travelers derailed in southern Pakistan on Wednesday, killing at least 56 people and injuring another 150, officials said. The train was speeding from Karachi toward Lahore when about 12 of its 16 carriages came off the rails near Mehrabpur, about 400 kilometers north of Karachi. It was unclear what caused the accident, which left hundreds of terrified survivors to claw their way out of the mangled wreckage in total darkness. By midmorning, rescuers had brought 56 bodies to three nearby hospitals, said Mumtaz Ali, an official from the Edhi Foundation, Pakistan's largest privately run emergency service. Col. Abbas Malik, an army doctor, said about 150 people were injured. An uncertain number of passengers, some of them crying out for help, were still trapped in the most badly damaged carriages, which had been thrown down a steep embankment into a waterlogged field. Rescuers were using a rail-mounted crane and cutting equipment to try to reach survivors in one blue-and-white carriage where the impact had thrust some of the seats through the roof. Dozens of soldiers and police helped tend the injured and carry them away to waiting ambulances, as hundreds of people from the surrounding villages looked on. The train, which derailed at about 2 a.m. (2100 GMT Tuesday), was loaded with some 700 passengers, many of them heading home for the Islamic holiday of Eid ul-Adha. Shahid Khan, a 25-year-old who had been traveling to Lahore with six of his relatives, said he used the light from his mobile phone to find his way out. "The train was going at full speed. Then there was a sudden jerk and we felt the train sinking into the earth. There was chaos everywhere," said Khan, sitting next to bundles of luggage he had salvaged from a carriage lying on its side in the field. He said stranded passengers lit bonfires and drank tea brought by nearby residents to ward off the cold. Khan had cuts on his hands and said one of his sisters suffered a broken arm. Mohammed Khalid, a railway official who was traveling in one of the rear wagons which stayed on the rails, said he suspected a problem with the track - possibly sabotage - caused the accident. "My guess is that there was some piece of rail was missing and the engine jumped the missing track and the following wagon got stuck," he said. After the crash, a section of one of the rails had been torn loose. The engine came to a halt about 1.5 kilometers further up the line. Deadly accidents are a regular occurrence on Pakistan's colonial-era railway network. A speeding train struck a crowded bus at a railway crossing near Lahore in October, killing 12 people and injuring about 50 others. About 130 people died in July 2005 when three trains collided in southern Pakistan.