Getting between Eastern Michigan and Southwestern Ontario by rail - a brief summary:
There used to be 4 (or more accurately 3 1/2) routes. Starting from the north is the CN main line from London, ON, via Sarnia, ON, and Port Huron, MI. It is CN's primary route for freight and is mostly double-tracked except for the St. Clair tunnel, which can accommodate double-stacks and Superliners. It was the route of the Amtrak International.
Next is CN's route to Windsor, or more specifically Walkerville, ON, where the Windsor station is located. It used to run further south along the Detroit River to ferry docks that continued the rail service to Detroit proper. This route sees much less freight traffic since the demise of the auto industry in SW Ontario, is mostly single-track after splitting with the route above and is used by VIA services between London and Windsor. The area between it and the tunnel under discussion here is densely occupied and getting to the tunnel would require a back-up move and an interesting routing that would add significantly to running times.
CP's main freight line is next, connecting to the tunnel they eventually acquired from Michigan Central/Canada Southern. This is the topic of this thread, since there is currently no passenger service on the route. What is now the CP tunnel, emerges in Michigan adjacent to Michigan Central (Ford) station downtown. The station on the Canadian side was destroyed by fire. Here is the tunnel entrance (from Wikipedia):
View attachment 26719
Finally was the former New York/Michigan Central route across the top of Lake Erie. This was the route used by NY Central and Amtrak for trains to Detroit from upstate NY and evolved into a "co-operative" venture between several railroads, known as the Canada Southern or CASO/CanSo - depending on which history you read. It owned the tunnel above. The track was abandoned in sections, with parts operated by short lines, some track pulled up and the rest falling into disrepair.