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Rescuers work Tuesday at the site of the train derailment near the village of Malaya Vishera, about 500 kilometres northwest of Moscow. ((NTV, Russian Independent Television/Associated Press))

Russian authorities have opened a terrorism investigation into a homemade bomb blast thatderailed an express passenger train, injuring dozens.

"A criminal case has been opened under Article 205, Clause 3 —that is terrorism," Sergei Bednichenko, chief prosecutor for Russia's North West district, told Channel One television Tuesday.

Twenty-five people were hospitalized after the blast struck the train en route from Moscow to St. Petersburg around 9:30 p.m. local time Monday, causing the locomotive and a dozen passenger cars to derail, said Russian railway officials. About 35 others were treated at the scene.

Russia's security agency tied the bombing to insurgents in southern parts of the country around Chechnya, where violence has spiked in recent months.

"The threat of terrorism and extremism has not yet been eliminated," said Nikolai Patrushev, head of the Federal Security Service, who consulted with President Vladimir Putin about the blast, news reports said.

Crews worked Tuesday to repair the nearly one kilometre of track damaged in the blastwith hopes of reopening it by the afternoon.

The blast happened near the city of Novgorod, about 485 kilometres northwest of Moscow. About 250 passengers were on board at the time.

The bomb exploded about 30 metres before the train crossed a bridge over a small river, according to news reports, but the train went off the tracks after crossing.

While the busy Moscow-St. Petersburg route is popular among foreign tourists, there were no reports of foreigners being among the injured.