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.
"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".