Take the train to Montréal
The VIA Rail network connects many major cities to Montréal, and it is by far the easiest way to access downtown. You can choose from 17 daily departures from the Gare centrale, which also gives you access to the Bonaventure métro station, taxi, car sharing, bus, BIXI bike rentals and the Underground Pedestrian Network. You can bring your bike on certain VIA Rail trains too.
If you are coming from the United States, the Amtrak trains offer daily departures from many American cities to downtown Montréal.