The City Circle is a mostly-underground railway line located in the Sydney central business district and Haymarket, in New South Wales, Australia, that forms the core of Sydney's passenger rail network. The lines are owned by the Transport Asset Holding Entity, a State government agency, and operated under Transport for NSW's Sydney Trains brand. Despite its name, the City Circle is of a horseshoe shape, with trains operating in a U-shaped pattern. The constituent stations of the Circle are (clockwise): Central, Town Hall, Wynyard, Circular Quay, St James, Museum and back to Central.

The Sydney central business district (CBD) is the historical and main commercial centre of Sydney. The CBD is Sydney's city centre, or Sydney City, and the two terms are used interchangeably. Colloquially, the CBD or city centre is often referred to simply as "Town" or "the City". The Sydney city centre extends southwards for about 3 km (2 mi) from Sydney Cove, the point of first European settlement in which the Sydney region was initially established.