Railways in Sydney on 9 December 1926 with the first suburban electric service running between Central station and Oatley on the Illawarra line. In the same year, the first underground railway was constructed north from Central station to St James in Sydney's central business district. Electric trains that had previously terminated at Central station continued north, diving underground at the Goulburn Street tunnel portal, stopping at Museum station and then terminating at St James. Other lines were soon electrified. Also, in conjunction with the construction of the Sydney Harbour Bridge which opened in 1932, an additional four-track underground line was constructed from Central
Sydney Harbour Tunnel South) to the Sydney Harbour Bridge, which has eight lanes, with a tidal flow operation. In 2008, the tunnel was carrying around 90,000 vehicles per day. The tunnel is made up of three sections: twin land tunnels on the north shore, twin land tunnels on the south shore and a immersed tube (IMT) structure. The tunnel falls about from the northern entrance and about from the southern entrance to its deepest point, below sea level. The IMT structure consists of eight precast concrete units. The units were constructed over away in a casting basin at Port Kembla and then towed