From Pau, the line follows the Gave d'Aspe valley, reaching Bedous (altitude ) in a relatively flat terrain. After this it climbs into the Pyrénées Mountains, with an average gradient of (2.5 %) to Etsaut, and then (4.3 %) to the tunnel at Somport, where the line reaches its maximum altitude of . To reach this point from Bedous, the steeply climbing line traverses both sharp curves, and passes through 14 tunnels including the helical tunnel at Forges-d'Abel.