The U.N.-funded Radio Okapi said 30 to 40 people were believed dead in the accident, in which a number of cars flipped over in the Central African country's Kasai Occidental province - between the city of Ilebo and the provincial capital of Kananga.
Officials could not be immediately reached for further information.
Radio Okapi said the accident appeared to have been caused by a technical problem, but did not provide details.
Roads and rail are notoriously dilapidated in vast Congo - a country the size of Western Europe only starting to emerge from decades of war and corrupt dictatorship.
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