"It seems there are over 60 dead in this accident," Koloso Sumaili, governor of Maniema province, said in Kinshasa after speaking with local officials in his region.
He said the accident happened approximately 200km south of the town of Kindu, as the train travelled towards the southern town of Lubumbashi.
Mr Sumaili said that people sitting on top of the train, as often happens in
African nations and other poor countries, were swept into the air when some of their luggage snagged on the beams of the bridge across the Lufulu river.
"Apparently, it was piles of goods they had stacked too high that caught on
the bridge, falling off and taking passengers with it," he said.
Mr Sumaili said that traders frequently took their goods from one place to another riding on trains.
"Trains that travel on that line have lots of people and goods on the roof. When the train was crossing over a bridge, the beams supporting the bridge swept people and goods off the train and into the river below," he said.
Congo's infrastructure is in tatters after years of decay followed by two wars over the last decade.
Most transport is now by air or river as roads and railways across Africa's third-largest country have crumbled or been swallowed up by the thick jungle.