There really could be a small book or at least a series of articles written about Detroit - Toronto service. Disregarding trains like the Amtrak International, which as mentioned used a different route for all of its existence, there is a long history of trains crossing between Windsor and Detroit - including other Amtrak trains in its early days. It sounds like a natural market and a great fit for restored/new service - until it isn't. Here's some background which I hope is at least informative:
Windsor and Detroit are about 40 minutes by air from Toronto with frequent, if overpriced, service. (Yes Windsor actually has an airport with significant service despite its proximity to DTW. There was a plan to actually cede their ATC to Detroit, but that proved very political for the federal government and was withdrawn in April.) There is also a major interstate-style highway (401) between Windsor and Toronto - a major trucking route that actually carries US I-75 branding near Windsor. There are few major cities on the route, with the westernmost (closest to Windsor) being London - a major university city with some industry. CN (and VIA) have two routes between Toronto and London - the more southerly is a fairly straight line used by the majority of CN freight and most of VIA's passenger trains. Once outside the GTA (Greater Toronto Area) it does not have a major passenger-generating stop until London, but is well-maintained and faster than the northern route. The northern route between Toronto and London hits a number of of major centers, but is significantly more circuitous and hosts only a couple of VIA trains - usually continuing to Sarnia, ON. (It always seemed like a curious choice for the International, but that's another chapter.) West of London, CN's tracks fork with the heavy double-tracked main heading north to their tunnel under the St. Clair River at Sarnia to Port Huron, MI. The main customer on the mostly single southern track to Windsor is VIA Rail. Having traversed this part of the route many times, it is not a fast trip. CP has its own tracks between Toronto and Windsor that host passenger service (GO) as far west as Milton, ON, with plans to extend to Cambridge - the site of a major Toyota plant. Beyond that CP is freight-only.
In 2017 the Government of Ontario announced with great fanfare the establishment of a High Speed rail link between Toronto and Windsor. While the proposal had a lot of people scratching their heads and a price tag in the stratosphere, it did address two problems which I alluded to above - connecting the major centers (including a developing high-tech corridor) on the north route between Toronto and London that had infrequent and slower VIA service and the route between London and Windsor, which had decent VIA service slowed by a lack of TLC by CN. I won't detail the whole proposal here, but here's a link for those interested:
High speed rail. The included map may be especially useful in understanding the connections. That government was defeated in the 2018 election, being replaced by one from the opposite end of the political spectrum. However, despite the suggestion of more fiscal responsibility, the new government preserved the plan with several changes. Most notably high-speed electric was downgraded to "higher speed" diesel, the route was extended to Detroit and CP joined the discussion as a participant. The newly-created transit oversight agency Metrolinx also became involved. (They are the "parent" of heavy rail GO services, as well as other regional transit services.) Due to some prior involvement with the Ministry of Transportation, I was invited to participate in a study which consisted of a tea-and-cookies presentation, reading through a detailed booklet and completing a 20+ page questionnaire. (That is why I have some limited knowledge of the proposal.) Of course the pandemic came along and other than a generic (and infrequent) newsletter I have no idea what is happening. It may be dead, since all levels of government that had an interest have depleted finances due to Covid.
Southwestern Ontario has always been the poor stepchild of VIA's Ontario/Quebec corridor. There likely isn't a pressing need to increase service in the foreseeable future. Whether they'd like to get rid of the route I don't know, but there are some similarities between it and Toronto - Niagara Falls for example. The latter saw less and less service until the joint Maple Leaf was the only remaining train. If there had been a market for more service VIA certainly could have continued with at least one train that offset the Maple Leaf in the opposite direction. Such a service did exist, but a number of factors that have been covered elsewhere led to its discontinuance. GO had seen an opportunity pre-Covid and had expanded service on the route significantly, so even when the Maple Leaf was suspended (first for bridge repairs, then the pandemic) there was a way to get to Niagara from Toronto. As has been discussed previously, GO had long-term plans to actually cross the bridge to connect with up to 3 Amtrak trains daily - ideally across the platform (required track work). The new Niagara Falls, NY, station was even built with provision for Canadian authorities to do pre-screening and the Ontario government had sought to purchase the entire route from CN. All pre-Covid of course.
Back to the Toronto - Windsor route, there have been some recent positive developments. The north route between Toronto and London has seen financial investment by the province, as it is a key route for GO expansion and all-day service - including the first electrified test section of the GO network. The CP line out of Toronto has also seen a lot of attention, including double track, increased passenger frequencies and planned expansion. Without these investments neither would really have been a desirable route for long-distance, but for various reasons both have improved potential.
In conclusion, a partnership between Amtrak and the Ontario Government is a possibility not mentioned in
@jis' comprehensive list, and it could work with more than one of his suggestions. If VIA is a participant that's great too, but I sense there might be room for both serving different markets. Maybe the Amtrak service doesn't have to serve Windsor at all or a platform on the CP route with a connecting shuttle would work. There would always be an option for connections at Chatham or London. Only time will tell.
