EDUCED by the distinctive architecture planned around the World Trade Center site, it is easy to forget that one of the most important transformations of Lower Manhattan will be almost invisible, at least from the street.A series of passageways being planned, including underground concourses, would permit pedestrians to make a sheltered journey, uninterrupted by traffic, from Vesey Street and North End Avenue in Battery Park City to Broadway and John Street. By paying a subway fare, travelers bound for the No. 2 or 3 trains could extend the trip as far as William Street; almost four-fifths of a mile under...