The Blue Mountains Line is an intercity rail service operated by NSW TrainLink serving the Blue Mountains region of New South Wales, Australia. The line travels west from Sydney to the major town of Katoomba and on to Lithgow on the western foothills of the mountains. 2 Express services per day in each direction, known as the 'Bathurst Bullet extend to the regional city of Bathurst, which is supplemented by road coaches connecting Bathurst to Lithgow. The Blue Mountains Line operates over a mostly duplicated section of the Main Western line. As such, the tracks are also traversed by the Central West XPT, Outback Xplorer and Indian Pacific passenger services and by freight trains.

The Blue Mountains line is a section of the Main Western line which opened in 1868. The line was built with gradients as steep as 1 in 33 (3%) and curves as sharp as 8 chains (160m). Most of the curves were eased to 12 chains (240m) with duplication.

The line originally ascended the eastern and descended the western sides of the Blue Mountains via a series of zig-zag track sections. The eastern zig zag was by passed by a tunnel in 1892 and the western zig zag (once a tourist railway) was bypassed in 1910 with the Ten Tunnels Deviation.

In the 1950s, the line was electrified primarily as a means of easing the haulage of coal freight from the western coalfields to the coastal ports, 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. Goods trains are now exclusively diesel hauled. Electric passenger services were originally provided by a combination of electric locomotive hauled carriages and single deck electric multiple unit sets (known as U sets), both of which have now been withdrawn and replaced by more modern rolling stock.

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