Trains to northern Quebec were hauled out through the Mount Royal Tunnel by ancient CN Electric Boxcab until 1990. After VIA transferred the xCP service to Quebec City (via Trois-Riviere) from Windsor Station to Gare Centrale you could also find a single RDC unit being hauled by a boxcab. Trains were hauled as far as Gohier (Eastern Jct. - the jct with the St-Laurent Subdivision) where the electric cut off.
After 1990 outbound VIA trains started going ‘round the mountain but inbound trains continued to use the tunnel for a couple more years until it was closed for major rebuilding of the infrastructure. There is a 0.6% downgrade through the tunnel from Portal Heights Station (now Canora) to Gare Centrale so trains would run under diesel power as far as the tunnel entrance, place the diesel in idle and just coast the 3 miles downgrade through the tunnel into Centrale.
Click on the Montreal Map in the list here:
The route VIA’s trains to northern Quebec follow now begins at Central (shown as Montreal in the centre of the map) Follows the blue (CN) line west to Taschereau Yard where it joins the St-Laurent Sub. then north-east across Montreal Island to Pointe-aux-Trambles. The dashed blue line north from Central Station is the Mount Royal Tunnel (Deux-Montagnes Sub) Eastern Jct is where it crosses the St-Laurent Sub.
Today’s route is about 15km longer but not much different than the route CP trains to the north and east to Quebec City always followed. Red route on the map.