The train was passing through Punjab city of Rahim Yar Khan on Thursday morning when the cylinder exploded, causing a fire that spread on the train, according to local police officer Amir Timor.
Seventy people are dead and another 30 are injured, said Nadeem Zia, a medical supervisor at Liaqatpur District Hospital, a city in Rahim Yar Khan district.
Earlier, Timor said that about a dozen people were critically wounded; Army troops, paramedics and a helicopter in Army aviation are currently on site.
Although gas cylinders were banned from trains, passengers used cookers to cook breakfast in the train wagon when the explosion occurred, added Timor.
He added that many of the people on the train were protesting in the Pakistani capital Islamabad, some 800 kilometers (500 miles) from Rahim Yar Khan.
The train was running along the Tezham line, ie. daily service that runs from the coastal town of Karachi to the northern city of Rawalpindi.