Three people were killed and 15 injured in an explosion in a compartment of Shalimar Express at Chatiana Railway Station near Toba Tek Singh Monday afternoon.
The train was going from Lahore to Karachi.
Chief Minister Punjab Shahbaz Sharif has condemned the blast.
The injured were shifted to District Headquarters Hospital Toba Tek Singh.
The explosion occurred in cabin number seven of the economy class. Two cabins have been damaged.
According to Shalimar Express officials, first aid boxes are available within the train and train managers are assisting the rescue work.
Trains have been filled up to capacity due to the upcoming Eid holiday season.
District Police Officer Asif Shehzad said three injured victims are in a critical condition at the hospital.
Railways Minister, Khwaja Saad Rafiq, said the initial investigation has been started. He said that a total 14 passengers were injured, four of them critical. He did not confirm death of any passenger.
He said the investigation suggested that the bomb was planted in the washroom of the train.
The police had told the train operator to take security measures in view of the threats, the minister said.
Saad Rafiq said a call was made to the owner for extortion and this is the work of those people. He said the Intelligence Bureau had been informed about the threatening calls from Afghanistan. He did not blame any group for the blast.
The minister said that the railways ministry has taken security of all trains with sniper dogs to check explosives.