While the Sydney Harbour Bridge is a steel through arch bridge, the Sydney Harbour Tunnel is what?
Sydney Harbour Tunnel: The Sydney Harbour Tunnel is a twin-tube road tunnel in Sydney, Australia. Sydney Harbour Bridge: The Sydney Harbour Bridge is a steel through arch bridge across Sydney Harbour that carries rail, vehicular, bicycle, and pedestrian traffic between the Sydney central business district (CBD) and the North Shore.
a twin-tube road tunnel