The blaze occurred after an explosion on the train near the town of Rahim Yar Khan in the south of Punjab province.
The train named Tezgam Express was travelling from Karachi to Rawalpindi when the incident happened. Some passengers were cooking breakfast when the gas cylinder exploded, the report said.
"Fire erupted everywhere", one survivor Muhammad Imran said in a hospital in Rahim Yar Khan.
He also informed that the train track would be cleared for commute within two hours, the Tableeghi Jamaat members were headed for a religious gathering in Rawalpindi and were trying to cook something over a gas cylinder when a possible leakage resulted in catastrophe.
Ghulam Abbas, a passenger who had got on the train in the town of Nawabshah in neighbouring Sindh province with his wife and two children, told of how they had watched panicked passengers in his carriage jump off.
Officials have said many more people are injured, some in a critical condition. Army troops including doctors and paramedics were also dispatched to assist rescue teams in the operation, a statement by the Inter-Services Public Relations said.
Pakistani soldiers and officials at the scene where a fire onboard a train killed a reported 74 people.
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Pakistan prime minister Imran Khan tweeted: "Deeply saddened by the bad tragedy of the Tezgam train".
Explaining the reason behind the fire, Pakistan's Railway Minister Sheikh Rashid Ahmad said two gas stoves carried by some passengers blew up when they were making breakfast on the speeding train, in violation of rules.
Minister for Human Rights Shireen Mazari tweeted that it was an avoidable tragedy because "ever since I can recall while travelling by train no baggage check or restrictions enforced".
Khan said that he had ordered the launch of an immediate inquiry to be "completed on an urgent basis".
Why was a gas canister on the train?
Pakistan has a history of deadly train accidents stemming from poor railway infrastructure.
In July, 11 people died in an accident, with four dying in another accident in September.