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.
2015: AMS ATL DEN DPS EWR FNJ FRA GFK GIG GRU GUM GVA HNL HKG IAD IAH ICN JFK KSA KWA KWL LIS MAD MAJ MCO MDW MNL MSN MSP NRT PEK PHL PNI PVG SFO SHA SJC SIN STL TKK XIY | CA DL G3 JQ LH LX NX OZ SQ TP UA UX WN
Location: Home:HKG/YYZ Business:YVR, LAX, SFO, SEA, PVG, PEK, CAN, NRT, MNL, SIN, BKK, KUL
Programs: DL girl's best friend, Lifetime(DL's) SC, IC RA, SPG, AARP, Nexus/GE, APEC(HK)
PRC passport with US green card is good, show the onward boarding pass or reservation.
Check through to PVG depends on the inbound and outbound airlines. I heard you need to check in your bags with Hainan Airlines.
Morning rush hour to downtown by taxi or limo is 45min+.
Shopping centres open 9:30am to 10am so the only option to kill time in a mall is Yorkdale, even that is stretching your timing for the 90minutes international checkin requirement. It should take no more than 30 minutes to get there and 15 minutes to get back(post-am-rush).