The line uses the Mount Royal Tunnel and Canadian National track from Montreal's Central Station to Repentigny. New track was built from Repentigny to Terrebonne along the Quebec Autoroute 640, before turning towards the Trois-Rivières Subdivision of the Chemins de fer Québec-Gatineau at Mascouche. Most of the route, in particular the section after Mont-Royal to Repentigny, follows a similar path to the never-built Line 6 of the Montreal Metro, which was planned as a steel-wheeled "regional metro" line using a somewhat similar alignment. The journey time between Mascouche and downtown Montreal (using the pre-2020 alignment before the closure of Mount Royal Tunnel) was 61 minutes.