Sorry Tom, while the tunnel you describe would be wonderful and probably a big traffic relief, the geology of that part of the island precludes it happening. It's the same reason the train is planned to be above ground through town. The route you describe is an area  that in the Pleistocene era was coral reef. It is very porous and the cost of tunneling and lining the tunnel would be hugely, bigly, prohibitively, impossibly expensive. You know, like train-expensive.