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.

Despite its name, the City Circle is of a horseshoe shape, with trains operating in a U-shaped pattern.

The original concept for the City Railway was part of a report dated 1915 submitted to the government by chief railway engineer, John Bradfield, upon his return from overseas study,[1] with work commencing the following year.

In 1956, the dead ends at St. James and Wynyard were joined and the "missing link", Circular Quay – was opened.

The current service patterns generally consist of trains from the Inner West & Leppington Line running into the City Circle Outer, becoming Airport & South Line trains as they enter the circle.

This allowed these stations to deal with 42 trains per hour in either direction provided sub 40 second dwell times.

Signals on the City Circle can no longer display a calling-on aspect, but can display low-speed (red over red over small green - which means that the block is clear but to proceed slowly, not exceeding 25 kilometres per hour (16 mph)).