More than 30 people were hospitalized, mostly injured by shattered glass, after 12 of 13 cars and the locomotive of the four-hour Nevsky Express went off the rails at 9:38 p.m. Moscow time (5:38 p.m. GMT). The train travels at a speed of 200 km/h (120 mph).
The injured included two foreigners, but they have been discharged from the hospital, a hospital official said.
The Russian Railways monopoly has said a terrorist attack was the only possible explanation, but law enforcement authorities are considering several other versions, including technical failure, particularly in the wake of a series of recent train crashes across Russia.
"There is no reason to say that it was definitely a terrorist attack," a law enforcement official said. "That version is being considered together with others."
One of the train attendants told RIA Novosti he had heard clapping sounds. "I remember two clapping sounds and then the train began turning inside out, I remember screeching that continued I think four seconds as the train was braking," he said.
A law enforcement official in the Veliky Novgorod Region said the suspected bomb might have contained two kilograms of TNT. "Bomb experts working at the site said a blast went off on the rails, and that it had the force of two kilograms of TNT."
The attendants helped passengers get out of the cars through emergency hatches and broken windows, and provided them with first aid. They said passengers did not panic and many of them helped the attendants.
Russian Railways said it would restore traffic on the affected section of the railway by 3 p.m. Moscow time (11 a.m. GMT), the company said in a news release.