John Bradfield’s plan for Sydney’s underground rail lines through the city included a Harbour Crossing, a City Circle line and an Eastern Suburbs line. The first was completed with the construction of the Sydney Harbour Bridge in 1932, but the Great Depression and World War 2 put the others on hold until after Bradfield’s death. The rise of the motor car after World War 2 meant these other two lines were not completed till 1956 and 1979 respectively. The City Circle was actually mostly completed in the early 30s, with Town Hall/Wynyard on the Western end of the CBD and Museum/St James on the Eastern End. Eventually, these were united in 1956 with the construction of Circular Quay Station.
Before this, Circular Quay was a significant interchange between ferries (it was and still is the major ferry terminus in Sydney) and trams. This can be seen in the photos above. The state government had been phasing out trams in the 1940s and 50s, replacing them with buses, the final tram running from Circular Quay down Anzac Parade to La Perouse in 1961, only 5 years after Circular Quay Station was completed. You can see footage of this last tram in the video below. The removal of trams from Sydney’s streets is regretted by many today, particularly by those pushing for their return, but it did have one benefit in this case. Bennelong Point, on the Eastern end of Circular Quay, was used as a tram shed up until 1961. Now a vacant lot, it was decided to build a great cultural icon there in place of the old tram sheds. The result was the Sydney Opera House. Should trams ever return to Sydney, I highly doubt that the Opera House will make way for the tram shed that used to be there.
The Cahill Expressway was later built over Circular Quay Station, allowing cars travelling Southbound on the Harbour Bridge to bypass the CBD when travelling to its Eastern end. There have been proposals to move the Cahill Expressway underground in order to reclaim it as public space (such as this proposal) or a bus interchange (to relieve the struggling Wynyard bus interchange).
Today, Circular Quay is the only station on the Cityrail network (to my knowledge) that provide free wifi. Ironically, the ferries (which are located right next to the station, and whose wifi you could probably connect to from within the station) also have free wifi, and from what I hear provide a much better connection.