<<The only train is the Via continuation of the Maple Leaf.>>
Not entirely.
GO Transit has a weekday commuter train leaving Niagara in the early morning and returning from Toronto late evening. (The remainder of the all- day Niagara Falls service is the bus/train link at Burlington.) GO also has weekend through train service to and from Niagara Falls in the summer, beginning on the May 24 long weekend and ending at Canadian Thanksgiving.
It is very easy to get from Niagara to Toronto. In addition to GO Transit there is frequent service on both Greyhound and Megabus.
Incidentally GO does not describe itself as being an inter-city service. It is a regional rail and bus service in the Great Toronto and Hamilton Region.
Finally, while "GO" stands for Government of Ontario, the service is actually operated by Metrolinx, a coordinating agency for transit in the GTA. It also operates the Union Pearson Express to and from Toronto Airport.