Upvote:3
There are plenty of options to get to Niagara Falls from the Greater Toronto Area (GTA). There is also one major change since the answers to the previous questions were posted: there is now one daily roundtrip GO train between Union Station and Niagara Falls.
If you're arriving in Toronto around 3pm on a weekday, you have plenty of time to take the UP Express train to Union Station, followed by the Niagara-bound GO train, which leaves Union Station at 5:15pm, and arrives in Niagara Falls at 7:47pm (see Table 12 here). While this isn't necessarily the fastest route, it's definitely the most comfortable and consistent at that time of day.
There are more GO trains to/from Niagara Falls on weekends on Victoria Day weekend, during the summer, and on Thanksgiving weekend (in 2019: May 18-20, June 22βSeptember 2 and October 12-14). The only other rail option is the VIA train, which leaves Toronto at 8:20am, and Niagara Falls at 5:45pm (see schedules here). There is no VIA train that calls at Burlington like in your question.
As I'm sure you've discovered, there are other transport modes too (bus, rental car, etc.). In summary:
By Rail (use UP Express to access Union Station: 25min, $12.35 cash, $9.25 Presto)
By Bus
Driving (call 511 or visit 511on.ca for real-time traffic conditions)
Though I don't make the trip often, I've traveled between Toronto and Niagara Falls around 20 times by various modes. I never regretted taking public transit (despite the extra time commitment), but nearly every time I drove, I regretted it eventually.