Russia's domestic intelligence service, the FSB, says the deadly derailment of an express train was caused by a bomb.
The luxury 14-carriage Nevsky Express was carrying 682 passengers and 29 crew when it derailed on the busy main line between Moscow and St Petersburg on Friday night.
The Emergency Ministry says at least 26 people have been confirmed dead, with another 18 missing, and 96 people have been injured, Reuters reports.
FSB head Alexander Bortnikov said experts believe an improvised explosive device, equivalent to 7kg of TNT, had gone off.
At least three of the 14 carriages left the tracks as the train reportedly approached speeds up 200 km/h near the village of Uglovka, about 350km north of Moscow.
No one has so far publicly claimed responsibility for the incident.
The attack, Russia's worst outside the turbulent North Caucasus since a series of 2004 suicide bombings in Moscow, has stoked fears of a surge in attacks in Russia's historic heartland by Islamist rebels from Russia's southern flank.
A second, less powerful bomb exploded about 2pm local time on Saturday at the site of the crash, though no one was injured, the head of the state railway operator said.