The Eastern Suburbs & Illawarra Line (numbered T4, coloured azure blue) is a commuter railway line in the eastern and southern suburbs of Sydney and is a part of the Sydney Trains network. The line was constructed in the 1880s to Wollongong to take advantage of agricultural and mining potentials in the Illawarra area. In March 1926 it became the first railway in New South Wales to run electric train services.

Today the railway consists of three connected lines:

Operationally and historically, the entire line from the Illawarra Junction at Redfern to its terminus in Bomaderry on the South Coast was known as the Illawarra Line. However, since 1989 CityRail has marketed the suburban services to Waterfall and Cronulla as the Illawarra line and interurban services south to Wollongong and Bomaderry as the South Coast Line. The line is coloured an azure blue on Sydney Trains timetables and other promotional materials.

The Suburbs Line runs between Bondi Junction in Sydney's east and Eveleigh, just south of the Sydney central business district. It is mostly underground, and consists of 7 kilometres of bored tunnels and 1.5 kilometres of cut and cover tunnels, with only 2 kilometres above ground. In the Eastern Suburbs, three tunnels proceed in a westerly direction from Bondi Junction via Edgecliff and Kings Cross; in each of these tunnels there are stations. Between Bondi Junction and Edgecliff there is a short open-air cutting in Woollahra, and between Edgecliff and Kings Cross there is a short viaduct over Rushcutters Bay.

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