Sixty-five feet below the streets of Manhattan, workers are digging the city’s first new subway line since the 1940s. The Second Avenue subway, to be named the T line, will eventually stretch from 125th Street in East Harlem to Hanover Square in the financial district. The first stretch of the line, from 96th Street to 63rd Street, is set to open in December 2016, carrying more than 200,000 passengers every day.