The accident occurred when the train was speeding too fast before turning around a bend close to the Katongola bridge, which is near the station at Katongola, a town in the southern Katanga province.
The train, which reportedly carried hundreds of passengers, was travelling from Kamina, a town in Katanga province, to Mwene-Ditu in the province of Kasai-Oriental, one of the richest diamond producing regions in the world.
A survivor said that the train derailed on a slope to the level of the Katongola bridge.
Most of the victims were tradesmen and travellers, including children, said Katongola station chief Jean Ngoy Mozadi.
The death toll was feared to rise because some victims were still trapped in the overturned cars, officials said.