This article says that a new Detroit River tunnel could start construction next year. From what the article and the official website about the project have to say about the project, there is absolutely nothing that would preclude passenger trains from using the tunnel. If anything, the Continental Rail Gateway could actually encourage passenger service as trains to and from Buffalo and Toronto could just as easily use the existing tunnel without a hitch.
A Plan Like This Could Use Some New Players
Speaking of international service, it wouldn't be necessary for either Amtrak or Via Rail to run these trains--as a matter of fact, other companies would be tapped to run Detroit-Windsor services instead. Amtrak built a facility in the city's New Center section in 1994, which would force the carrier to back up to the Detroit River Tunnel or to skip the city altogether. As for Via, the Canadian operator has cut back service on the Toronto-Windsor corridor. A Eurostar or Thalys-like service would remedy the problem.
The State's Role
The Midwest Regional Rail Initiative plan has the Pere Marquette rerouted to serve Kalamazoo, turn west to serve Grand Rapids, and then end in Holland. Should that happen, then, the MIDOT should hand the current Chicago-Grand Rapids route over to another operator like First Group or Veolia, rename the route the Spartan Corridor in honor of the state's other big public university (think about it: Wolverines vs Spartans leaving the gridiron and hardwood for the rails with travelers winning as a result), extend the route to Detroit, and change the stations at each endpoint. There would have to be at least four or five frequencies for the rebranded Spartans in order for the trains to compete with the Amtrak trains.
LaSalle Street Station would be a perfect place for the new operator to compete against
Amtrak because it wouldn't have to deal with the congestion of Union Station. At the other end, a temporary station could be built near Michigan Central Station until the historic facility is once again ready for public use.
The Spartan trains would then use the Detroit River Tunnels to go east to Buffalo or northeast to Toronto as a way for the new operator to produce more revenue and ridership. In the future, SNCF would use the tunnels for Express service between Chicago and Toronto.
What About Other Cities in the Wolverine State?
When it comes to Detroit-Wynadotte-Monroe-Toledo service, it is a natural, but, it would require a backup move at a wye just west of MCS. Would Amtrak want to operate a glorified commuter route that is only 58-miles long--two miles longer than the Lake Cities route that ran two decades ago--and would also require a backup move near the New Center Station? I think not. Even a Port Huron-Toledo route that serves either Detroit station would be too short. Maybe, the airport would be an alternative for Amtrak or a northwestern stop for the Spartan Corridor operator.
As for other cities in Michigan, the MIDOT should turn to the state's various regional and shortline railroads to operate services. These trains would easily connect to Amtrak and Spartan Corridor trains in the same way that the Saratoga & North Creek trains do with the Adirondack and Ethan Allen Express at Saratoga Springs.