You seem to imply that there was a connection to the tunnel west of Walkerville. Those tracks never went past Goyeau Street downtown. The only connections west of Walkerville to anyplace else were the ferries.
CN passenger trains never used the tunnel. They terminated at Goyeau St. until 1961, then at Walkerville until VIA started. Cross-river CN passenger trains only went on the ferry to the GTW slip, and they quit doing that in 1955.
The Niagara Rainbow used the PC/CR CASO all the way to the border. It did not operate on CP tracks anywhere, except possibly joint trackage through Welland. The Windsor station had always been owned by CR and its predecessors until the sale of the CASO to CP/CN in 1985. CP's Windsor passenger trains had relocated to a remodeled CP yard office in 1968, with the last run on July 3, 1971.
Part of the CN line that its road freights use to get into Windsor, less than 3 miles long from Jefferson Blvd. to Pillette Rd., could be upgraded to take a passenger train from the CP up to the VIA line. A simple bus-shelter type station could be built at the site of the old MC Windsor depot to get service started. Or, the Essex Terminal could be upgraded, also less than 3 miles long. Or a passenger train could stay on the CP all the way to Chatham or beyond. These are entirely feasible projects. But I'm not holding my breath.