The accident occurred near the southwestern city of Quetta, capital of Baluchistan province, after the train's brakes failed as it sped down the side of a mountain, officials told AFP, with many of the injuries believed to be serious.
"Four carriages of a Rawalpindi-bound train which left Quetta Railway station were derailed near Abegum area of Bolan district, some 75 kilometers (46 miles) from Quetta city," Akbar Hussain Durrani, home secretary of Baluchistan, told AFP.
"The incident happened because the brakes of the train failed," said Durrani.
Faiz Bugti, a senior official of the railways department, and head of local community police Tariq Zehri also confirmed the accident and casualties.
Durrani said a rescue and relief operation had begun and officials had already arrived on the scene.
Train accidents are common in Pakistan, which inherited thousands of miles of track and trains from the former colonial power, Britain.
The railways have seen decades of decline due to corruption, mismanagement and under-investment.
In July, at least 17 people were killed when a special military train fell into a canal after a bridge partially collapsed.