The bridge here is the main bridge to the Imperial Palace, called by at least one source “arguably the most famous bridge in Japan”.  Most people call it Nijūbashi but the correct name is 正門石橋  (main entrance stone bridge).  There are actually two main bridges, to the Imperial Palace. The 正門石橋 seimon ishibashi (main entrance stone bridge) and the 正門鉄橋 seimon tetsubashi  (main entrance iron bridge).   In any event here’s an old postcard of 正門石橋 , unused and Grade: 1