The STM is replacing the oldest cars in its fleet, but the second-generation metro cars will have to keep running until 2036.
The Montreal transit agency said it cannot afford to replace the MR-73 cars which came into service between 1976 and 1980.
It estimates maintaining the metro cars will cost $108 million over the next two decades, while replacing the 423 cars would cost $1.9 billion.
Meanwhile the oldest metro cars, the MR-63s, are being replaced by Bombardier-Alstom.
The STM has expanded its order, and is now calling for a total of 540 Azur cars at a cost of $1.7 billion.