Any Tunnel, could be similar to how the channel tunnel works, with freight wagons that carry cars, and lorries, on a shuttle operation, plus passenger trains, that would have their own terminal, depending on where the tunnel came out, if near Ryde , a short spur could link into a new station on Island line so there is an interchange,
If the tunnel was over on the west wight, then then one option might be to use the old track bed from Yarmouth to Newport and Cowes with new stations, at Newport and Cowes? That's assuming there was the money and will do it. to bring the islands capital, and at least one entry point, onto a fixed connection with the mainland.