State TV said five of the train's 11 carriages had tipped over in the early-morning accident.
Iranian media reported that arrest warrants had been issued for six people, without identifying them or giving details on their alleged crimes.
Jafar Miadfar, head of Iran's National Medical Emergency Organisation, said on television that so far 18 of the dead passengers had been identified. All but six of the injured had been released from hospital, he said.
President Ebrahim Raisi declared Thursday a day of national mourning.