6 line. What is the string for? Pull string for emergency? Pull string for to get an MTA employee on the line? NYC The Blog will follow this up, and updates will be added with any new information.
UPDATE: After NYC The Blog took to Twitter to query the MTA about the string, they responded almost immediately, double checking first before explaining the string, is "being used to pull wire through the conduit to this Customer Assistance Intercom installation." Soon, that box will indeed get you through to someone, though via phone instead of string.