The accident occurred near Quetta city, the provincial capital, after the train's brakes failed as it sped down the side of a mountain, officials told AFP.
A military spokesman in Quetta said that 26 seriously injured passengers were airlifted to hospital in two helicopters.
"Four carriages of a Rawalpindi-bound train which left Quetta railway station were derailed near Abegum area of Bolan district, some 75 kilometres [46 miles] from Quetta city," Akbar Hussain Durrani, Baluchistan home secretary, told AFP.
"The incident happened because the brakes of the train failed."
Faiz Bugti, a senior railways department official, and head of local community police Tariq Zehri also confirmed the accident and casualties.
The army has joined a rescue and relief operation and will use heavy machinery to free victims, while a quick response force from the paramilitary Frontier Corps has also been deployed, authorities said.
TV footage showed rescuers trying to pull the injured from the derailed cars.
Railway officials said the train was carrying around 300 passengers.
"Among the dead so far confirmed, five are passengers, two drivers of the train and two members of the railway police," Durrani said.
Train accidents are common in Pakistan, which inherited thousands of miles of track and trains from former colonial power, Britain.
The railways have seen decades of decline due to corruption, mismanagement and lack of investment.
In July, at least 17 people were killed when a special military train fell into a canal after a bridge partially collapsed.