We already have the Shuttle and #7 subway line. Do we really need another crosstown rail line?
The light rail line will be a different type of service, reaching waterfront development and ferry terminals, which neither the Shuttle nor subway can do. The Shuttle will remain useful for connection between the 7th Avenue and Lexington Avenue subways, but the light rail will have a much farther range. As a surface transit system, it is also more convenient for local trips; whereas the #7 subway is 80 feet below grade, requiring a lot of stair climbing.