At St Marys, the line becomes two tracks as it passes through Penrith and Emu Plains, the extent of Sydney suburban passenger train operation.

The first "little" zigzag line opened near Glenbrook in 1867 as part of the ascent of Lapstone Hill on a gradient of 1 in 30–33.

It involved extremely heavy rock cuttings, three fine stone viaducts with 30-foot (9.1 m) semi-circular arches and a short tunnel.

[3] From the western foot of the Blue Mountains, the line was promptly extended to Wallerawang by 1870, Tarana in 1872, Bathurst, Blayney in 1876 and Orange in 1877.

[4] The town of Bourke had become the key centre for pastoralists in western New South Wales since its formation in 1861.

At the time, Dubbo had grown into a town of strategic importance on the stock routes between northern New South Wales and the Victorian goldfields.

Beyond Dubbo, the railway opened up new land to European settlement, and was directly responsible for the development of townships.

[6] In the 1950s, the section of the line over the Blue Mountains was electrified primarily as a means of easing the haulage of coal freight from the western coalfields to the coastal ports,[7] but a by-product of this programme was the introduction of electric interurban passenger services as far west as Bowenfels, later cut back to the current terminus of Lithgow.

[11] In the 1990s the operator of interstate freight, the National Rail Corporation, made the decision to divert Sydney- Perth traffic from the Blue Mountains section, to travel via the Main South line to Cootamundra, and then via the cross country line to Parkes.

This resulted in reduced goods traffic and subsequent reduction of the line between Wallerawang and Tarana from double to single track.

The section to Lithgow carries electric commuter trains to and from Sydney (the Blue Mountains Line).