Spanned by five movable-truss bridges dating from 1875 to 1931, the navigable Fort Point Channel, which separates downtown from the South Boston Seaport, has been called “The Museum of Bridges.” The oldest are the swing span Broadway Bridge (1875, altered 1902 and 1915) and the South Station Old Colony Bridge (1898, New York, New Haven, & Hartford Railroad), a Scherzer six-track rolling lift bridge. Two bridges draw on designs invented by T. Willis Pratt: the Summer Street Bridge (1900), a paired retractile draw, and the Old Northern Avenue Bridge (1904–1908), a triple-barreled truss swing span designed by Boston City Engineer William Jackson. The much altered Congress Street Bridge (1931), an electrically operated Strauss bascule overhead-counterweight drawbridge, replaced an 1874 steam-operated iron swing bridge. Desmond and Lord designed its granite piers topped with lanterns. All but the Old Northern Avenue Bridge are now stationary, as channel traffic is limited to small fishing and pleasure boats. Late in the 1990s, Boston built Moakley Bridge (Ammann and Whitney), two spans of prestressed I-beams and three spans of concrete box girders, next to the Old Northern Avenue Bridge as its replacement. Preservationists continue to battle Mayor Thomas Menino to rehabilitate the old bridge for pedestrians, as “a walkable link to Boston's future,” in the words of urbanist Jane Holtz Kay.