Northern Light
Superstar
Also - the express GO schedule to Niagara (i.e. the one that doesn't reverse in to West Harbour) runs in 2:15, not 2:36 like the peak hour service.
Remember as well that GO has a lower top speed than VIA / Amtrak services, which can operate at 166km/h when GO tops out at ~144km/h.
You end up with Amtrak doing Union-Aldershot in 36 minutes which takes GO 52 minutes to complete - almost the entirety of the extra travel time. After Aldershot, it takes GO 1:23 vs. Amtrak's 1:19 to get to Niagara Falls - very similar.
Yes to all of that.
But let's note one thing on that very last comparison, that the Maple Leaf has one extra stop on that segment, at Grimsby. Presumably, but for that, the time difference would be a bit greater.