Toronto Transit operates a bus from Union Station to Niagara Falls every 10 minutes. Tickets cost $3 and the journey takes 8 min. 2 other operators also service this route. Alternatively, VIA Rail operates a train from Union Station to Niagara Falls every 3 hours. Tickets cost $30 - $45 and the journey takes 4 h 15 min.
From Front St. At Bay St. line 6 bus to Toronto, ON then bus to Niagara Falls, ON. 2 h 28 min $22. From Toronto train to Niagara Falls. 4 h 15 min $49. From Union Station drive to Niagara Falls. 1 h 30 min $16.