The DNA tests were required to identify as many as 52 charred bodies before they could be handed over to relatives for burial, Dawn news quoted Jamil Ahmed, the deputy commissioner of Rahim Yar Khan, as saying on Friday.
The tragedy on Thursday left three coaches of the Rawalpind-bound Tezgam Express destroyed and killing at least 74 people.
Most of the victims were travelling to attend the annual Tableeghi Ijtema, one of Pakistan's biggest religious gatherings, which each year sees up to 400,000 people descend to a village outside Lahore for several days to sleep, pray and eat together.
The train was carrying 857 passengers, including 550 members of the preaching group. The fire apparently started in two compartments where Tableeghi Jamaat members were onboard.
In Sindh, where most of the victims were from, authorities handed over several identified bodies to relatives for burial on Friday in the town of Mirpurkhas and elsewhere.
Other relatives were seen lining up to give blood samples for DNA tests outside a hospital in Rahim Yar Khan.
Ahmed said that the forensic teams hope to complete the identification process within 48 hours, adding that authorities were investigating the incident, including why it took the train so long to stop.
According to survivors, it took nearly 20 minutes for the train to stop following the fire.
( With inputs from IANS )