Iran is observing a national day of mourning for victims of the train disaster which killed at least 295 people in the country's north-east.
The situation on the ground is still dangerous
Revolutionary Guards have cordoned off the area near the town of Neyshabur in case more damaged petrol wagons ignite.
Buildings in at least five nearby villages were devastated by the massive explosion, and more victims are believed to be buried in the rubble.
It is still unclear what caused the heavily-laden freight train to derail.
The government has ordered an immediate inquiry into the disaster and has demanded the results by Sunday.
Iranian TV said the freight train - which included 17 wagons of sulphur, six wagons of petrol, seven wagons of fertilisers and 10 wagons of cotton wool - broke loose and rolled about 20km (12 miles) down the track.
Officials said as many as 450 people were injured - many of them with severe burns - and local hospitals were overwhelmed.
Correspondents report that acrid fumes from the toxic combination of chemicals have spread for kilometres around the crash scene.
Firefighters battling in the freezing cold overnight finally extinguished the blaze shortly after dawn on Thursday, the Associated Press reports.
Local residents have been moved away, with officials warning there could be further blasts.
The train wagons derailed and caught fire at 0400 (0030 GMT) on Wednesday.
Some 200 of the dead are thought to be rescue workers who were tackling the fires when the explosion happened at 0935 (0605GMT).
Blast like quake
The blast was so powerful that local residents thought it was an earthquake - and Iranian seismologists recorded a quake of magnitude 3.6 at the time of the explosion.
Officials want to know how the freight wagons, which were not attached to a locomotive, could have simply moved off down the line, eventually careering off the track and bursting into flames.
The explosion left hundreds of people injured
They are also demanding to know why the train was loaded with such a lethal combination of chemicals.
Iran is still recovering from a devastating earthquake on 26 December in the ancient city of Bam in the south-east of the country, in which more than 40,000 people died.
WORLD'S WORST TRAIN DISASTERS
June 2002: Dodoma region, Tanzania - at least 200 killed when passenger train collides with goods train
Feb 2002: Egypt - 300 killed in fire on train travelling to Cairo
June 1989: Ufa, Russia - More than 400 killed in gas explosion under two trains
Aug 1995: Uttar Pradesh, India - 300 killed in train collision
June 1981: Bihar, India - 800 killed when cyclone blows train into river
"The whole town is shocked by this accident," the editor of a local newspaper, Saeed Kaviani, told AP news agency.
"Official vehicles mounted with loudspeakers are roaming the city calling for volunteers to donate blood."
The blast shattered windows more than 10km (six miles) away and could be heard in the regional centre of Mashhad, 75km from the scene.
Top city officials, including the local governor, mayor and fire chief, are believed to be among the dead.
The Iranian news agency (Irna) said the head of the city's energy department had also been killed and that the director-general of the provincial railways was missing.
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