A third line was planned.
It was to use Canadian National Railway (CN) tracks passing under the Mount Royal to reach the northwest suburb of Cartierville from the city centre.
Unlike the previous two lines, trains were to be partly running above ground.
Negotiations with the CN and municipalities were stalling as Montreal was chosen in November 1962 to hold the 1967 Universal Exposition (Expo 67).
Having to make a choice, the city decided that a number 4 line  (Yellow Line) linking Montreal to the South Shore suburbs following a plan similar to those proposed early in the 20th century was more necessary.

Line 3 was never built and the number was never used again.
The railway, already used for a commuter train to the North Shore at Deux-Montagnes, was completely renovated in the early 1990s and effectively replaced the planned third line.
The next line would thus be numbered 5 (Blue Line).
A third line was planned. It was to use Canadian National Railway (CN) tracks passing under the Mount Royal to reach the northwest suburb of Cartierville from the city centre. Unlike the previous two lines, trains were to be partly running above ground. Negotiations with the CN and municipalities were stalling as Montreal was chosen in November 1962 to hold the 1967 Universal Exposition (Expo 67). Having to make a choice, the city decided that a number 4 line  (Yellow Line) linking Montreal to the South Shore suburbs following a plan similar to those proposed early in the 20th century was more necessary.
Line 3 was never built and the number was never used again. The railway, already used for a commuter train to the North Shore at Deux-Montagnes, was completely renovated in the early 1990s and effectively replaced the planned third line. The next line would thus be numbered 5 (Blue Line).
Montreal Metro