Union Station is a large, busy Station, similar in usage to NYP or Chicago Union, that serves VIA, the Subway, Amtrak (Maple Leaf is Amtrak Equipment but uses VIA Crews between Toronto and Niagara Falls, ON where it becomes an Amtrak Train to NYP) and Go Trains and Buses.
I would say you should arrive at least an Hour early (.no checked baggage) and suggest using a Red Cap to board the train since Union Station is hard to get around in if you don't know it well!
I also strongly suggest you ride in Business Class ( it's in a Split Cafe/Biz Class Cat with the nice 2x1.seating and you get priority on clearing US Customs and Immigration @ the Border) though technically VIA doesn't offer Biz Class on this Train, you pay $1 to ride from Toronto to the Border, then whatever the Amtrak upgrade costs from the Border to NYP in Biz Class!
You'll have two tickets, a VIA ticket and an Amtrak ticket. Definitely Worth it!
Edited by Bob Dylan, 08 August 2016 - 11:44 PM.