At least 73 passengers died when a train travelling from the Pakistani city of Karachi to Rawalpindi caught fire.
Minister for railways, Sheikh Rashid Ahmed, said the fire was caused by the explosion of a gas cylinder being used by passengers cooking breakfast.
The blaze is thought to have spread to at least three carriages.
According to officials cited in local media, many of the victims died as they tried to jump off the burning train.
Another 40 people have been injured. Officials say the number of dead may still rise.
The accident happened near the town of Rahim Yar Khan in the south of Punjab province. Many passengers were pilgrims heading to Raiwind near Lahore for one of Pakistan's largest annual religious congregations, organised by the Tablighi Jamaat Sunni Muslim missionary movement.
"Two cooking stoves blew up. They were cooking, they had [cooking] oil which added fuel to fire," Sheikh Rashid Ahmed said.
Passengers bringing stoves onto trains in order to cook meals on long journeys is a common problem, the minister said. Though it is common to carry food on board, gas cylinders are banned.
District deputy commissioner Jamil Ahmed said some of the victims were so badly burnt they could not be identified, and they would need to use DNA to find out who had died.
Prime Minister Imran Khan said he was "deeply saddened by the terrible tragedy" on Twitter, adding that he had ordered an "immediate inquiry".
The train running from Karachi, across most of Pakistan to Rawalpindi, is the Tezgam - one of Pakistan's oldest and most popular train services. It runs daily and takes 25-and-a-half hours.
After an earlier suspension in the wake of the fire, the railways minister said services have now been restored.
This is Pakistan's worst rail disaster in over a decade.
The country has a history of railway accidents with multiple deaths. Casualty figures are often so high because trains are packed with far greater numbers than they were designed for.
In July, 11 people died in an accident, with four dying in another accident in September.
In 2007, at least 56 people were killed and more than 120 injured in a crash near Mehrabpur.
And in 2005, more than 130 people were killed when three trains collided in Sindh province in one of the country's worst train disasters.
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