Local police officials said sabotage was probably the cause of the accident and that iron plates used to connect the rail lines had been removed.
The train was travelling from the southern port city of Karachi to the eastern city of Lahore when six of its coaches derailed near the town of Samarsatta in the central province of Punjab.
The crash came about a week after another express train came off its tracks last Sunday in eastern Pakistan, killing three people and injuring more than 70.
Police have arrested four scrap metal dealers suspected of cutting railway lines in the area.
Last July, at least 133 people died in a three-train pile-up in the southern province of Sindh.
In Pakistan's worst train crash, 307 people were killed in 1990 when a packed passenger train smashed into a freight train, also in Sindh province. A train crash in Punjab province killed 136 people in 1997.