"Following the rumours on the presence of nuclear materials in the explosion of the train wagons in Neyshabour, two nuclear physics experts were sent to the site and, after investigation, did not find any nuclear materials present," Mohammad Maghdouri, vice governor general of Khorassan province for development affairs, told the state news agency IRNA.
Last Wednesday, a string of 51 wagons loaded with chemicals, fuel and fertiliser derailed and exploded at Khayyam station near Neyshabour, some 75 kilometres (50 miles) from the northeastern city of Mashhad.
According to a new toll from the provincial officials, 289 people were killed in the blast which had an estimated force of 180 tons of dynamite.
Transport Minister Ahmad Khoram has blamed either human error or the actions of a disgruntled employee for the tragedy.
The United States charges that Iran is secretly developing nuclear weapons and has called for the UN atomic watchdog to refer the Islamic republic to the UN Security Council for possible sanctions.
Iran says its nuclear program is strictly peaceful.