Customs and Border controls.
The Province of Quebec is paying to move Customs and Border control to the Montreal Station. Like we have at Vancouver. This way the train will then run sealed to the US and not required any Customs and Border check at the actual border saving a 2 hour delay or so. The Maple Leaf going to Toronto has the 2 hour delay baked in, even when the actual Customs and Border checks were taken less than a hour.
Imagine sitting in station waiting your departure time after you have cleared Customs and Border. That going to be the most frustrating one hour of your entire trip.
The VIA Rail point is the transfer passenger. There a lot of passenger traffic that get on at Saints Lambert. It was a common way to connect from the east. They can try to do the Customs and Border off train at Saints Lambert but that going to be a cost issue.
I have never seen more than 2 people get on or off at St Lambert, and half the time, the northbound train blasts right through. There are no offical connections at all between Amtrak and VIA Rail. Neither railroad will hold their train. You are totally on your own if you try it.
All Quebec is doing is paying C$1 million - again - to draw up plans for Preclearance at Gare Central, nothing is budgeted to build it. If it ever happens, the train simply will not stop at St Lambert.
Since the Adirondack is not a VIA train north of the border, and does not handle local Canadian passengers, it can depart CBSA Lacolle northbound when they are done and arrive Montreal early.