Russian authorities say that a bomb caused the derailment of the Moscow-St Petersburg express late on Friday.
The driver and several passengers said that they heard a blast just before several carriages came off the track.
The Nevsky Express was packed with hundreds of passengers when the derailment occurred at peak time.
Dozens of people were killed and injured.
Many victims were trapped for hours in overturned carriages.
Medics set up a makeshift treatment centre at a nearby railway station.
The incident has caused long delays for passengers seeking to travel on this busy route.
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