Unless I was on a particularly slow train, the answer should be around 160 km/h between Montreal and Toronto. The highest was maybe 165 km/h. This is from a GPS that I've used in at least five cars and it's never been that far off from the vehicle's speedometer. I doubt they've swapped over to newer trains or have allowed faster speeds since these pictures were taken a few years ago.
VIA Rail Cruising-ish Speed
This was between 7:00am and 11:00am on a weekend morning. I seem to recall the train slowing down near road crossings. The regular cruising speed was closer to 125 km/h with stretches above 150 km/h being frequent, but not too consistent. Regardless, the train arrived on time.