The separatist militants blew up a railway track and opened fire on the Jaffar Express on Tuesday as it travelled from Quetta, Balochistan's capital city, to Peshawar in Khyber Pakhtunkhwa province.
Pakistani forces have rescued 155 passengers, and the government said a security operation was under way to free dozens still held hostage, without specifying the exact number.
The Baloch Liberation Army (BLA), an ethnic armed group, claimed responsibility for the attack and threatened to start executing hostages unless Baloch political prisoners, activists, and missing persons it said had been abducted by the military were released within 48 hours.
BLA said on Tuesday it was holding 214 people hostage, and a security source told Reuters that there were 425 passengers on the train when it was attacked.
The number of militants involved in the attack was not clear.Â
The security sources said on Wednesday that 27 had been killed so far.
BLA is the largest of several ethnic armed groups battling Pakistan's government in the mineral-rich province of Balochistan, bordering Afghanistan and Iran.
Several of those rescued were brought to Quetta early Wednesday, escorted by security forces, where their relatives were waiting for them.
"People were attacked ... passengers were injured and some passengers died," said Muhammad Ashraf, who was on the train.
Several witnesses interviewed by Geo News said they were asked by security personnel to stay low when there was gunfire.
Visuals from the broadcaster showed those rescued meeting and hugging relatives and friends.
A woman, who said her son was among the passengers still held hostage, confronted provincial minister Mir Zahoor Buledi when he visited the freed passengers.
"If you cannot protect trains, then you should not run them. Please, bring my son back," she said.
Pakistan Railways has suspended all operations from Punjab and Sindh provinces to Balochistan until security agencies confirm the area is safe , local media reported on Wednesday.
Buledi told reporters that the government was working to improve the security situation in the region.
