At about from Nouadhibou, the Mauritania Railway originally passed through the long and steeply graded Choum Tunnel, to avoid passing through part of the Río de Oro territory in Western Sahara, which was then occupied by Spain. Since the end of the Western Sahara War in 1991, a diversion of the line has been built through the Polisario-controlled Free Zone of Río de Oro, thus enabling the closure of the Choum Tunnel.