Beginning on May 11, 2020, the Mascouche line was rerouted around the Mount Royal Tunnel, bypassing Mont-Royal and Canora stations due to the construction of the Réseau express métropolitain (REM).

The 51-kilometre (32 mi) line used 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 Autoroute 640, before turning towards the Trois-Rivières Subdivision of the Chemins de fer Québec-Gatineau at Mascouche.

On May 14, 2007, the former Agence métropolitaine de transport (AMT) and New Jersey Transit put out a joint call for tenders to purchase dual-powered locomotives.

On the Mascouche line (until May 2020), the locomotives switched to electric power at Mont-Royal (this was originally planned for Ahuntsic) to enter Mount Royal Tunnel to Central Station.

Stations going east along the CN St Laurent Subdivision from Eastern Junction where it meets the Deux-Montagnes line were: Ridership was never very high.

A temporary service dubbed the "Métropolitrain" was organized by the STCUM from May 15 to October 12, 1990, while Autoroute 40, the boulevard Métropolitain, was being rebuilt.

[5] There are 11 current stations on the Mascouche line:[6] Most observers agree that the proposed route from Montreal to Repentigny makes sense for several reasons, including the use of existing infrastructure.