Sunday's accident occurred near the town of Jhelum in eastern Punjab province at around 7.30pm (1430 GMT), said Salimur Rahman Akhund, general manager of operations with the state-run Pakistan Railways.
Akhund said the 10-car train went off the tracks and four carriages fell into a ravine.
Pakistan's Geo Television reported that six carriages tipped off into the 15-metre-deep gorge in the mountainous area.
Akhund did not have any information on casualties but estimated that between 500 to 600 passengers may have been in the train.
"We pray to God to keep everyone safe," Akhund said.
Soldiers and rescue teams reached the scene by late night with flood lamps and tents to provide basic medical treatment for survivors, General Shaukat Sultan, an army spokesman, said. Rescue efforts were hampered by the dark.
Carriages in ravine
A woman aboard the train, who called her family in Lahore from a mobile phone, said three or four carriages had plunged into the ravine, according to her husband, Mohammed Ashfaq, who spoke to The Associated Press by phone.
Pakistan suffered its worst rail
crash in a decade on 13 July
The woman, Yasmin Ashfaq, was travelling with her three children and none were hurt.
Passengers who suffered no injuries were being sent to a nearby town on a different train, she said.
People were crying and screaming after the accident, she was quoted as saying.
Hafeez Rahman, senior doctor at Jhelum Civil Hospital, said several ambulances were sent to the scene.
"We are prepared and ready to handle any emergency," Rahman said. "But so far, we do not know how many people are hurt."
The Islamabad Express train was headed to Lahore, the capital of Punjab, from Rawalpindi, a city near Islamabad, according to Akhund.
Pakistan suffered its worst train crash in a decade on 13 July 2005, when a train driver misinterpreted a signal, pulled into a station and slammed into another train.
More than 130 people died in that accident, near Ghotki, in southern Sindh province.
A train carrying 800 passengers from Karachi to Lahore slammed into a parked freight train, also near Ghotki, on 8 June 1991, killing more than 100 people. Authorities blamed staff negligence for that accident.
In December 1989, a train crash near Sangi, a town 35 miles from Ghotki, killed 400 people.