If you are a greencard holder or US citizen, you may not need the transit visa.
However for other US visas, the Airlines ask for the Canada transit visa.
We made a mistake last year booking via Frankfurt to Toronto to USA. They stopped us at Frankfurt and we booked a direct flight to USA only on the next day and stayed in the Frankfurt airport that night
Is a Transit Visa Required?
in IR-1 / CR-1 Spouse Visa Process & Procedures
Posted
If you are a greencard holder or US citizen, you may not need the transit visa.
However for other US visas, the Airlines ask for the Canada transit visa.
We made a mistake last year booking via Frankfurt to Toronto to USA. They stopped us at Frankfurt and we booked a direct flight to USA only on the next day and stayed in the Frankfurt airport that night