The order could be worth up to $481 million if the government buys all 140 cars. So far, it has placed firm orders for 65 of the two-level passenger cars and taken out options for another 75.
Montreal-based Bombardier (TSX:BBD.B) says the firm order is worth $190 million.
Deliveries are expected to begin in June 2016.
The provincial government says the order will support 250 jobs at Bombardier's plant in northwestern Ontario.
The new cars will be added to the GO Transit fleet, which serves commuters in Toronto and surrounding communities.