A line of 75 people stood outside the Trader Joe’s between Court Street and Atlantic Avenue in Brooklyn late one evening. But it wasn’t to pick up the latest in cookie butter or chimichurri rice. No, it was to sneak underground through a manhole under the dark cover of night. Every time the light changed at the intersection, groups of five scurried into the road to slip below Cobble Hill to get a glimpse of the world’s oldest subway tunnel.