The train's locomotive and several cars derailed late on Saturday "due to the collapse of a span structure of the road bridge as a result of an illegal interference in the operation of transport", Russian Railways said on the Telegram messaging app.
Alexander Bogomaz, the governor of the Bryansk region, said on Telegram that 44 people were hospitalised. Three children were among those injured with one in serious condition, he added.
Russia's ministry of emergency situations said on Telegram that efforts to find and rescue victims continued throughout the night, and that some 180 personnel were involved in the operation.
Among those killed was the locomotive driver, Russia's state news agencies reported, citing medics.
Social media pictures and videos showed passengers trying to help others climb out of the train's damaged carriages in the dark and firefighters looking for ways to reach passengers.
Russia's Baza Telegram channel, which often publishes information from sources in the security services and law enforcement, reported, without providing evidence, that according to preliminary information, the bridge was blown up.
Reuters could not independently verify the Baza report and there was no immediate comment from Ukraine.
Since the start of the war that Russia launched more than three years ago, there have been continued cross-border shelling, drone strikes, and covert raids from Ukraine into the Bryansk, Kursk and Belgorod regions that border Ukraine.
The train was going from the town of Klimovo to Moscow, Russian Railways said. It collided with the collapsed bridge in the area of a federal highway in the Vygonichskyi district of the Bryansk region, Bogomaz said. The district lies some 100km from the border with Ukraine.
US President Donald Trump has urged Moscow and Kyiv to work together on a deal to end the war, and Russia has proposed a second round of face-to-face talks with Ukrainian officials next week in Istanbul.
Ukraine is yet to commit to attending the talks on Monday, saying it first needed to see Russian proposals, while a leading US senator warned Moscow it would be "hit hard" by new US sanctions.
with Reuters
