[3] The railway leaves Paris-Montparnasse in southwestern direction for the first 3 km, and turns west at Malakoff, skirting the southern quarters of the city of Versailles.

It turns southwest again until Maintenon, where it starts following the river Eure upstream, passing Chartres.

It continues west through Guingamp and Morlaix until it reaches its terminus Brest, at a bay of the Atlantic Ocean.

The part between Viroflay and Chartres was opened in 1849, Chartres–La Loupe in 1852, La Loupe–Le Mans in 1854, Le Mans–Laval in 1855, Laval–Rennes in 1857, followed by Rennes–Guingamp in 1863 and Guingamp–Brest in 1865.

[2] The train protection system Contrôle de vitesse par balises (KVB) is operational on the Paris - Saint-Brieuc section.