Local television broadcast images of a huge column of black smoke and flames shooting into the sky from coaches with their windows shattered, as firefighters battled the blaze and rescue workers searched for victims.
At least 44 people were killed when two passenger trains collided and caught fire in northern Iran on Friday, officials said.
Mohammad Reza Khabbaz, governor of northern Semnan province, told Iranian television it appeared that a train entering the Haft-Khan station on the outskirts of Shahroud ploughed into another that had broken down there.
"The initial investigation suggests that a mechanical failure, possibly caused by cold weather, forced the express train, operating between the cities of Tabriz and Mashhad, to stop (at Haft-Khan)," Khabbaz said.
The accident is described as one of the country's worst train disasters,
Hossein Kulivand, head of Iran's emergency services, said late on Friday 44 people were killed and 82 hospitalised, of whom 17 were treated for light injuries and released.
Television broadcast images of a huge column of black smoke and flames shooting into the sky from coaches with their windows shattered, as firefighters battled the blaze and rescue workers searched for victims.
"I was sleeping when the crash happened. I thought it was an air strike ... When I opened my eyes, there was blood everywhere," a hospitalised passenger told state television.
Tabriz state governor Rahim Shohratifar told a local news agency that the moving train had 400 passengers. It was not clear how many passengers were on the stationary train.
The province's Red Crescent director, Hassan Shokrollahi, said the remote location of the crash site, between Semnan and Damghan, the next major town, had complicated rescue efforts.
"Due to the difficulty of access, only our helicopter has managed to reach the scene," he said.
The injured were airlifted to hospitals in Semnan and Damghan.
The Tehran-to-Mashhad line was briefly closed to allow an investigation into the cause of the crash, said Sadegh Sokri, spokesman for Iran's railways.
President Hassan Rouhani called for "all technical, administrative and preventive measures to be taken to prevent the recurrence of such an accident".
In the country's deadliest rail disaster, 328 people were killed when a train transporting sulphur, petrol and fertilisers exploded in northern Iran on February 18, 2004.