The militants blew up the rail tracks and opened fire on the Jaffar Express as it made its way to Peshawar in the province of Khyber Pakhtunkhwa from Quetta, the capital of mineral-rich Balochistan province, taking hostage several of the 440 aboard.
Insurgent group Baloch Liberation Army (BLA), the largest of the region's armed ethnic groups battling the government, claimed responsibility for the attack, which killed 21 hostages and four security troops.
Men armed with rocket launchers, guns, and other weapons stormed the train on Tuesday and began shooting people, said Arslan Yousaf, one of those who arrived in Quetta escorted by security forces.
The militants grouped the passengers on the basis of their region of origin, Yousaf said on Thursday.
"Sometimes, they took soldiers ... and executed them," he said, referring to passengers from the Pakistan army and other security forces who were travelling on leave.
"Other times, they targeted specific individuals. If they had a grudge against someone, they shot him on the spot."
Hundreds of troops, special forces and military helicopters joined the effort to free the hostages stranded in remote mountainous terrain.
The hostages survived only on water during the time they were held, said Muhammad Tanveer, another passenger.
Reuters video images showed the rescued hostages receiving first aid at the Quetta railway-station.
"The terrorists breached the train by smashing windows, but they mistakenly believed we were dead," said train driver Amjad, who dived to the engine floor for cover when the militants opened fire, and lay there for about 27 hours to survive.
The BLA had threatened to start killing hostages if authorities missed a 48-hour deadline to release Baloch political prisoners, activists, and missing people it said had been abducted by the military.
The group says the government denies the region its fair share of the benefits from its mineral resources.
Prime Minister Shehbaz Sharif would visit the area on Thursday, media said.
He condemned the attack in a post on X on Tuesday, adding, "Such cowardly acts will not shake Pakistan's resolve for peace."
