A train travelling from the Russian border town of Klimovo to the capital Moscow was derailed in the incident, authorities said, without giving details.
Videos posted on social media showed rescuers working at the site of a large mound of rubble covering what appeared to be a train belonging to national operator Russian Railways, while another showed people shouting in distress.
“There are seven dead as a result of the collapse of a bridge onto railway tracks,” Aleksandr Bogomaz, the Bryansk region’s governor, wrote on Telegram.
At least 69 others were injured, including three children, he added.
Moscow Railways, a state-owned subsidiary, said a passenger train derailed “between Klimov and Moscow due to the collapse of a road bridge span, as a result of illegal interference in the operation of transport”.
The incident happened at night between Pilshino and Vygonichi stations in the Bryansk region, the railway operator said on Telegram.
The incident did not affect other train traffic, the firm added.
In one video posted on social media, someone can be heard screaming as eyewitnesses rush to find help.
“How did the bridge collapse? There are children there!” a woman can be heard shouting in the video.
Photos published online by Russian authorities showed a collapsed section of the bridge and damaged vehicles, as rescue workers were deployed overnight.
The disaster area is around 100 kilometres from the Ukrainian border. (AFP)