A two-day mission to deliver 300,000 mosquito nets by rail to 10% of Congo's population has begun.
The United Nations says the shipment is part of the largest ever distribution of mosquito nets to combat malaria.
The train is travelling to the capital, Brazzaville, from Pointe-Noire across jungles, hills and savannah, where access to medical services is limited.
The disease is one of Africa's biggest killers and in Congo it is responsible for nearly a quarter of child deaths.
The United Nations Children's Fund (Unicef) and the Congolese government have hired the train to distribute the mosquito nets, which have been donated by the Japanese government.
The colonial-era Congo-Ocean railway is still the only viable route between Congo's port and the capital.
It was completed by the French in 1934 after 10 years of forced labour. An estimated 17,000 workers died cutting the path through the jungle - many from malaria.
After leaving Pointe-Noire, the train will stop in small towns along the way, where wagon-loads of nets will be dropped off for distribution free of charge in health centres and hospitals.
In a few weeks a further shipment of 200,000 nets will arrive.
In total there will be enough for every pregnant woman and child under five.
The government says access to a free mosquito net is a basic right for children.
Malaria kills more children under-five than any other disease in Africa.
The malaria parasite is spread by female mosquitoes, so by sleeping under these chemically treated nets people can avoid being bitten at night, when many mosquitoes are most active.