Russian authorities say two bombs blew up Sunday along railroad tracks in the southern republic of Dagestan, derailing a freight train in another apparent terrorist attack.
A blast on the line that links Moscow and Azerbaijan derailed the engine and eight cars full of construction material.
The second bomb went off when a rescue team arrived. No injuries are reported.
Investigators already are calling it an act of terrorism - the third such attack in Russia in less than a week.
Two suicide bombers killed 12 people Wednesday in Dagestan.
And two female suicide bombers blew themselves up in separate stations last Monday in the Moscow subway. Forty people were killed and 90 wounded.
Chechen rebel leader Doku Umarov claimed responsibility for the Moscow blasts and said more attacks are on the way.
Russian President Dmitri Medvedev has promised crueler and tougher measures against terrorists.
Authorities have identified one of the subway bombers as the 17-year-old widow of an Islamic militant from Dagestan.
On Sunday, a man from Dagestan told the Novaya Gazeta newspaper that his 28-year-old daughter was the second bomber. He says he recognized her from a photograph of the suspected bomber distributed on the Internet.
Authorities have not yet commented on the newspaper report.
Some information for this report provided by AP, AFP and Reuters.