The Baroque Liberation Army (BLA) fired on a Jafar Express train as it moved from Quetta to Peshawar.
The separatist group said it bombed the tracks before raiding a train in the remote Sibi district, claiming the train was under its control, the BBC reported.
Local media said at least 16 militants were killed and 100 passengers were released on Wednesday morning. The BBC was unable to independently verify these numbers.
Among those released are 17 injured who were admitted to hospital for treatment.
Local reports say that if authorities did not release a Baroque political prisoner within 48 hours, extremists threatened to kill hostages.
Rescue operations are underway.
A spokesman for the Balochistan government told local newspaper Dawn on Tuesday that there was a report of “severe shootings” on the train.
The officer was “stuck just before a tunnel surrounded by mountains,” the report added.
MA/PR