Canadian airports are moving to a model where passengers on a single booking can transit directly from their international arrival to US preclearance without going to Canadian immigration and customs. This has been rolled out at Montreal and most other Canadian airports.
From the airport website:
If your flight originated outside Canada or the United States and you need to take a connecting flight to a U.S. destination, follow the signs "Connections". There are two possible options:
If your airline offers the option of automatically transferring your luggage and you have a boarding pass for your connecting flight, you will be asked to proceed via a connecting flights corridor, avoiding Canadian customs. You will, however, have to go through U.S. customs. Your checked luggage will be transferred automatically by the luggage-handling system. Note, however, that U.S. customs officers are authorized to recall your luggage, if necessary, for manual inspection. If the customs officers do not yet have the information on your luggage at hand, you will be asked to proceed to the waiting area.
If your airline does not offer the option of automatically transferring your luggage, you must pick it up yourself from the carousel, then follow the normal procedures for international arrivals, then for U.S. departures.
For now, even though you might not need to pass through customs, you still must be admissible to Canada, either through visa exemption or by holding a (usually transit) visa.
Passengers in transit: Attention! Check whether you need a transit visa.
Citizens of certain countries and territories need a visa to enter Canada. If you are among them, you will need a transit visa to travel through Canada without stopping or visiting. This applies even if you are in Canada for less than 48 hours.
You may not need a transit visa if you are travelling to or from the United States. The Transit Without Visa program (TWOV) and the China Transit Program (CTP) allow certain foreign nationals to transit through Canada on their way to and from the United States without a Canadian transit visa, if they meet certain requirements.