Work is underway on the Hudson River Tunnel project – also known as Gateway. At $16 billion it's one of the most expensive transit projects in the country. The plan involves building a new tunnel under the Hudson River for NJ Transit and Amtrak trains and then, repairing the existing tunnel that was damaged during Hurricane Sandy. The project also presents plenty of engineering challenges. WNYC transit reporter Stephen Nessen joins Weekend Edition host David Furst to share what he learned on a tour of the project.