At least 43 people died Friday in a train collision in Iran's north-central province of Semnan. More than 100 others were taken to hospitals after the crash.
Iranian state television showed footage of four derailed carriages, with two of them on fire. Rescue teams were working near the burning carriages.
The accident happened on the main line between the capital, Tehran, and second city Mashhad, when a passenger train in motion hit an express train that had stopped at a station because of mechanical problems.
The provincial governor, Mohammad Reza Khabbaz, told state TV that the number of causalities is likely to rise and that cold weather may have been a factor in the crash.
"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," Khabbaz said.
But the Associated Press reported that a parliament representative from the region hinted that officials at the Semnan train station may have mistakenly allowed one of the trains to proceed prematurely.
Iranian President Hassan Rouhani issued a statement offering his condolences to the victims' families.
According to the Associated Press, Iran reports about 17,000 deaths on average in traffic accidents every year. The high accident rate is mostly caused by drivers disregarding traffic laws, old vehicles and inadequate emergency services.
Years of punitive international sanctions against Iran over its controversial nuclear program have seriously affected Iran's infrastructure, including roads and the railway network.