SHOULD I PREBOOK MY TRAIN TICKETS BEFORE I LEAVE CANADA..OR WAIT TILL I ARRIVE IN CHINA

I think it mainly depends on your travel route and date. During holiday time, you surely need to pre-book the ticket in advance. They sold out really fast. Also, if you are traveling a long distance trip, it is also advisable to book in advance. If for a short and there are many departures a day, it should be fine. You can buy it after your arrival in China.