Twelve carriages of the train came off the rails
Prosecutors say an improvised explosive device caused the accident, which injured at least 60 people. More than 30 were taken to hospital.
A crater was found at the scene, in the Novgorod region. The train was on one of Russia's busiest routes.
Railway officials say 800m (yards) of track was damaged. Some people were pulled out through broken windows.
Witnesses say several carriages overturned and fallen power lines crackled overhead.
Criminal probe
The crater was 1.5m in diameter and 2kg (4.5 pounds) of explosive was used, investigators said.
Investigators from the Federal Security Service (FSB), as well as the prosecutor's office, are at the scene.
The driver reportedly heard a loud bang before the train derailed, at 2143 (1743 GMT) on Monday.
"A criminal case has been opened under article 205, clause 3, that is terrorism," said Sergei Bednichenko, chief prosecutor for Russia's north-west district.
The engine and 12 carriages of the train came off the rails just as it was about to cross a river bridge, Russian Railways said on its website.
The incident occurred near the village of Malaya Vishera, about 500km (310 miles) north of Moscow.
Officials say 231 tickets had been sold for the express service - meaning it was only about half-full - and there were 20 railway staff on board.
Le train de passagers ''Neva Express'' reliant Saint-Petersbourg à Moscou a déraillé à 21h43 heure de Moscou (17h43 GMT) le 13 août 2007 après l'explosion d'une bombe près de la gare de ''Malaya Vishera''. Au moins 60 personnes ont été blessées, certaines ayant été conduites à l'hôpital, mais aucune tuée.
La bombe a créé un cratère d'environ 1,5 m de diamètre, indiquent les autorités. 800 mètres de voies et de systèmes d'aiguillages ont été endommagés. La bombe, faite maison selon les autorités, avait été déposée sous le pont.
Une enquête criminelle a été ouverte par les autorités russes. Elle a été liée à une rébellion dans le sud du pays autour de Tchéchénie et relève, selon le Service de Sécurité Fédérale, d'une campagne de violences planifiées.
Le trafic ferroviaire a été bloqué dans les deux sens. Les déplacements entre Moscou et Saint-Petersbourg ont été détournés via Volkhov.