[2][3] The service, along with the North Shore Line service (yellow), later also ran along the North Shore railway line towards the City via the Harbour Bridge, forming a continuous loop via Strathfield, Epping and back to Hornsby.

As a result, the traditional Northern Line was split in two, with trains from north of Epping operating via the new link, approaching the city via the North Shore Line and rejoining the traditional route before terminating at Epping.

As a result, the traditional Northern line branch from Hornsby to Central via Strathfield was reinstated.

Trains pass through Redfern and Central, then through the western limb of the City Circle before heading across the Harbour Bridge.

However, the route is split into two distinct stopping patterns during the morning and afternoon peak: Anti-clockwise direction: Clockwise direction: The route from the Northern Line to Redfern station is officially branded and published as T9, but from Central onwards, PIDS (Passenger Information Display Screens) show all services as T1, even if it came from the Northern Line.