Investigators and rescue workers are seen amid damaged coaches at the site of the derailment. (Ivan Sekretarev/Associated Press)

Investigators in Russia say they have found evidence that a bomb caused a train derailment that killed at least 26 people and injured dozens more.

Traces of explosives were found among the wreckage near the town of Uglovka, 250 kilometres northwest of Moscow, said Alexander Borotnikov, head of Russia's Federal Security Service.

Officials say three carriages of the 14-car Nevsky Express, travelling from Moscow to St. Petersburg, careered off the tracks Friday night as the train approached speeds of 200 km/h in a rural area.

"Criminal experts say that based on preliminary findings a bomb equivalent to seven kilograms of TNT was detonated," Borotnikov said.

Vladimir Markin, a spokesman for federal prosecutors, was quoted by the Interfax news agency as saying this was a "terrorist attack." However, no one has claimed responsibility.

Attacks on civilian targets have been a concern in Russia since the 1991 collapse of the Soviet Union, as Chechen rebels have clashed with government forces in two wars.

Igor Pechnikov, one of more than 600 passengers, described being in the second of the three derailed cars.

"A trembling began, and the carriage jolted violently to the left. I flew through half of the carriage," he said.

Another passenger, Vitaly Rafikov, told Channel One state television he heard a bang before the derailment, which left two cars overturned.

There are reports that investigators found a crater measuring one metre deep beside the track.

With files from The Associated Press