ISLAMABAD - Pakistani investigators began a formal probe on Thursday into the derailment of an express train, with officials saying a mechanical fault on the track was the likely cause of the crash that killed at least 40 people.

The Karachi Express night train, packed with holiday travellers, was on its way from the southern city to Lahore when most of its 18 coaches jumped the track early on Wednesday.

A senior railway official ruled out sabotage, saying a broken joint on the track appeared to have caused the crash as passengers headed home for the Muslim festival of Eid al-Adha, being celebrated on Friday.

"As of now, a technical problem is still the possibility," General Manager Railways, Asad Saeed, told Reuters by telephone

from the site of the incident near the town of Mehrabpur in the southern Sindh province.

"It will be premature to say any definite thing before the inquiry."

The report was likely next week after the Eid holidays.

The official death toll stood at 40 people on Thursday, although officials had earlier said 58 people were killed. Nearly 800 people were on board the train when it began its journey in Karachi.

More than 120 people were injured.

Faisal Edhi, an official of the Edhi ambulance service, which removed most of the bodies from the scene, said 37 dead were brought to different hospitals in Mehrabpur, but that there were about another dozen bodies collected by relatives at the site.

Authorities have removed the wreckage and Saeed said that the track would be restored on Thursday.

Most of Pakistan's railways date from the colonial era and there has been little investment or upgrades in new lines.

About 130 people were killed in July 2005 when a crowded passenger train rammed into another at a station, also in Sindh province, and a third train hit the wreckage.