Originally Posted by Seat13F_AC_CRJ

I would not attempt this connection.

You will need to go through Canadian Immigration, exit to landside, take the train to to T3, security, gate, etc. If you were doing a USA-Intl connection all on AC, you would be able to skip Canadian Immigration.

More info here: http://www.torontopearson.com/Connecting.aspx#
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13F