the 1990s, it was often necessary to observe a "power margin" to ensure that substations were not overloaded. This situation was similar to that which applied to The Milwaukee Road's 3,000 VDC electrification. Plans to electrify the Hunter Valley at 25 kV alternating current were abandoned in the 1990s. Electrification came to Sydney's suburbs 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 station to Town Hall and Wynyard. Two of the tracks continued over the Harbour Bridge connecting to the North Shore line. It was only in 1956 that the two tracks terminating at Wynyard were linked to the line terminating at St James via the Circular Quay viaduct. This new arrangement, dubbed the City Circle, allowed services to make a continuous run through the city and return to the suburbs without having to terminate. Electrification timeline 1926 – Illawarra line to Royal National Park, Bankstown line, City Circle 1927 – North Shore line 1929 – Western line to Parramatta 1929 – Northern line (Strathfield to Hornsby) 1929 – South line to Liverpool 1936 – Carlingford line from Clyde to Rosehill 1955 – Parramatta to Penrith 1957 – Penrith to Lithgow 1959 – Rosehill to Carlingford 1959 – Hornsby to Cowan 1960 – Cowan to Gosford 1968 – Liverpool to Campbelltown 1975 – Blacktown to Riverstone 1980 – Loftus to Waterfall 1982 – Gosford to Wyong 1984 – Wyong to Newcastle 1985 – Waterfall to Port Kembla. 1985 – Macarthur station opened 1991 – Riverstone to Richmond 1996 – Coniston to Dapto 2002 – Dapto to Kiama New electrified lines were built: 1926 – line from Central to St James station and Museum opened 1931 – line opened to East Hills 1932 – line from Central to Town Hall, Wynyard, the Sydney Harbour Bridge, Milsons Point station and North Sydney station opened 1939 – Line to Cronulla built