The 500-kilometre Congo-Ocean line, which links the Congolese port city of Pointe-Noire to the capital, Brazzaville, was built between 1921 and 1934.
The French colonial administration had contracted the work to the Société de Construction des Batignolles, which later merged with the Spie group.
They used conscripted labour and an estimated 17,000 people died from accidents, poor work conditions and disease.
The labourers were not criminals sentenced to forced labour but civilians, according to historian Olivier Le Cour Grandmaison.
“This concerns civilians,” he told RFI. “Spie rounded them up to exploit them. These forced labourers were put into what were called ‘rail camps’, which were displaced as the construction progressed. Death rates in some of these camps were up to 57 per cent.”
France abolished forced labour over a decade after the work was finished, in 1946.
Slavery was recognised as a crime against humanity in a 2001 law.
The Representative Council of Black Associations (Cran) filed a suit on Tuesday against Spie, two other companies and the French state.
The association’s president, Louis-Georges Tin, says besides compensation for the victims’ descendants, he’d like to see the companies contribute to the communities affected:
“It would be useful to build schools, hospitals, structures in all the villages that suffered so much from this,” he told RFI.
The Cran fights for reparations for the descendants of slavery.
In May it sued the French Caisse des dépôts bank, accusing it of having profited from the slave trade.