The journey across the desert takes about 17 hours and covers 704 kilometres - roughly the same as travelling from London to Edinburgh and back again. The train runs through some of the most remote places on Earth, with nothing but sand, rock, and open sky on every side.
Iron ore is a mineral found underground that contains iron - a metal used to make everything from bridges to bicycles to skyscrapers. Mauritania has enormous deposits of it, especially around the town of Zouerat in the far north. Mining iron ore is one of the most important industries in the country.
Here is the most surprising thing: passengers are allowed to ride the train too - not in smart carriages, but by climbing into the open wagons on top of the ore itself. Travellers from all over the world have made this journey, wrapped in warm clothes and scarves to protect from flying dust, watching the Sahara slowly pass beneath a sky blazing with stars.
The train is so heavy that when fully loaded it takes a very long time to slow down - engineers must start braking far in advance. At the port of Nouadhibou, giant machines tip the wagons upside down to pour the ore into ships that carry it to factories around the world.