The accident occurred after the brakes failed as the train traveled between the city of Ilebo and the provincial capital of Kananga, said Medard Ilunga, head of Congo's state railway agency.
Seven cars overturned and an eighth went partly off the tracks just before midnight Wednesday, about 100 miles northwest of Kananga, Ilunga said. The conductor was able to detach the locomotive and go for help.
The accident "resulted in a heavy toll of about 100 dead," government spokesman Toussaint Tshilombo Send told The Associated Press.
Dozens of people were injured, but officials did not provide a specific figure or give any details of the wounds.
Rescue workers had pulled at least 70 bodies out of the wreckage by late Thursday, but other victims were still trapped under the rail cars, according to a report on the country's U.N.-backed radio station, Radio Okapi.
Injured passengers were being carried on people's backs and on bicycles to a hospital six miles away, Radio Okapi said.
Send said the government sent crews to the crash site to help in the rescue effort and comfort the families. He added that an investigation will be mounted into the accident.
In addition, the U.N. has sent helicopters with doctors, nurses and medical equipment to help, said Kemal Saiki, spokesman for the U.N. peacekeeping operation in the Central African nation.
The United Nations has a large, established force in Congo to help the embattled country in its transition to democracy following decades of war and corrupt dictatorship.
Roads and rail lines are seriously dilapidated in Congo, a country the size of Western Europe. Most of Congo's railroads were built more than 100 years ago, when the country was a Belgian colony.