If this train originated in Detroit instead of Chicago-NYP via Canada, would it be feasible - or reasonable - for passengers to clear Canadian customs before even setting foot on the train? This is the practice at NYP for the Adirondack to Montreal, I believe.
Now, that might not address the reverse problem of wily Canadians somehow boarding the train and sneaking into the U.S....
No, that was not the practice on the Adirondack. At NYP, Canada bound passengers had their ID checked and got special baggage tags, that was all. The Adirondack stopped for customs inspection right at the border and customs inspection took place onboard while the train sat there. That was in both directions.
For the record, customs and immigration formalities were handled differently for each of the cross border trains.
Adirondack Onboard inspection at border, as mentioned. All Amtrak operation.
Maple Leaf - ALL passengers deboard with all luggage at Niagara Falls, ON (north/west bound) and Niagara Falls, NY (south/east bound) for customs and Immigration inspection inside station. Wait in station until train itself is cleared. New(ish) Niagara Falls, NY station built with joint use by both CBP and CBSA in mind if Canada exercises her Preclearance rights, but right now , no. Joint Amtrak/VIA operation. Amtrak crew between Niagara Falls, ON and New York, VIA crew between Niagara Falls, ON and Toronto.
Cascades - Northbound customs and Immigration inspection on arrival at Pacific Central Station. Southbound US Immigration inspection prior to boarding at Pacific Central Station in a "Preclearance light". US Customs inspection onboard train at border in Blaine,WA. With recent rail preclearance agreement, that is expected to change at some point with both US immigration and customs being done at Pacific Central Station. All Amtrak operation.
I would expect a Maple Leaf model for a Chicago-Detroit-Toronto service, since it would most likely be making Canadian stops and also be a joint Amtrak/VIA operation. Border formalities would be done using either a joint CBP/CBSA facility in Detroit's MC station with Canada exercising preclearance rights, or CBP in MC and CBSA in Windsor.
I do not think a New York-Chicago route via Southern Ontario is in the cards at all, and in the small chance it were, it would not run sealed. Border formalities would be done in Detroit/Windsor and Niagara Falls.
Finally, neither CBSA nor CBP like onboard inspection and probably would not permit another one. That is one of the reasons the Adirondack is likely ultimately to switch to all border formalities for both sides being done in Montreal, much like Vancouver.