This is an exciting possibility that could mesh well with Ford's Michigan Central plans. And it would not just be good for Detroit-Toronto but for an efficient direct route to and from NYC.

In the seventies I took a train back from NYC to Detroit. I rolled up the Hudson Valley to Albany, then followed the old Eire Canal route across upstate NY, crossed at Niagara, passed through southern Ontario without stopping, then recrossed via the tunnel and emerged directly into Michigan Central.

I vaguely recall some sort of onboard customs check.