The only train ride at YYZ is the
LINK between T1, T3 and the Viscount parking lot. It's free too, so if all you are looking for is a train ride, you do have time for a few trips and it won't cost you a penny. Nothing much of interest to see in T3 or at the parking lot, so might as well stay on the train and go back to T1.
If you want to go downtown, your time is very limited. Inbound immigration and outbound security can take a while at YYZ. Downtown is a 30+ minutes away even by taxi, so you probably would only have an hour or so downtown if all goes well. If your inbound flight is early and/or immigration is quick, you will hit the roads at peak rush hour traffic. If your flight is late or immigration is slow, there will be less traffic but you are already late to begin with. No good solution, really.