An explosive device caused the derailment of the Nevsky Express travelling from Moscow to St. Petersburg, Russia's federal security chief said Saturday.
At least 26 people were killed when three carriages of the train came off the tracks Friday evening near the town of Bologoye in the Russian region of Tver, some 160 km from the Russian capital.
The toll is expected to rise, with unconfirmed reports of up to 39 deaths.
"Preliminary information indicates that improvised device equivalent to seven kg of TNT exploded," said Alexander Bortnikov, the Russian federal security chief. A criminal case has been filed by the prosecutors on charges of terrorism.
The train, which can travel at 200 kmph, was carrying some 650 passengers and 29 railroad employees.
"An unidentified explosive activated by unknown people went off," said Vladimir Yakunin, president of railroad monopoly Russian Railways (RZD).
Russian President Dmitry Medvedev has instructed Emergency Situations Minister Sergei Shoigu to take all necessary measures to help the affected people.
Chechan terrorists had set off a blast on the same route in August 2007, injuring 60 people in the accident that followed.