Iran detained three railway officials on Saturday after a collision between two trains that left at least 44 people dead, the official IRNA news agency reported.
"During the investigation, three people in charge of the northeast train control centre based in the city of Shahroud... were arrested," provincial prosecutor general Heydar Asiabi told reporters.
Two trains collided and one caught fire in the northern province of Semnan on Friday, killing 44 people and injuring dozens more, in one of the country's worst ever rail disasters.
The crash took place on the main line between Tehran and Iran's second city Mashhad.
One of the trains had stopped between the towns of Semnan and Damghan after an apparent mechanical failure, forcing officials to halt others on the line.
But when a new shift started at the control centre in Shahroud, the second train was allowed to resume its journey, Hossein Ashouri, an Iranian Railways Company official, told state television.
Supreme leader Ayatollah Ali Khamenei offered his condolences on Saturday for the "painful incident", which came just a day after more than 60 Iranian pilgrims were killed in a suicide attack in Iraq.