|Mount Gambier railway line|
|System||South Australian Railways|
|Continues from||Adelaide-Wolseley line|
|Continues as||Millicent line|
|Opened||Wolseley-Tatiara: 18 April 1883|
Tatiara-Naracoorte: 21 September 1881
Naracoorte-Mount Gambier: 14 June 1887
|Closed||12 April 1995|
|Operator(s)||South Australian Railways|
|Line length||183.2 km (113.8 mi)|
|Track gauge||Pending Standard Gauge 1,435 mm (4 ft 8 1/2 in)|
The Mount Gambier railway line was a railway line on the South Australian Railways network. Opened in stages from 1881, it was built to narrow gauge and joined Mount Gambier railway station, which was at that time the eastern terminus of a line to Beachport. It connected at Naracoorte to another isolated narrow gauge line joining Naracoorte to Kingston SE, and to the broad gauge Adelaide-Wolseley line at Wolseley, at around the same time that was extended to Serviceton to become the South Australian part of the Melbourne-Adelaide railway. There has been regular calls for its standardisation between Wolseley and Heywood since its closure over the past two decades, which was complicated by the severing of the two lines in 2015.
An isolated line was authorised by the South-Eastern Railway Act in 1871 and completed in 1876 from the port at Kingston SE inland via Lucindale to Naracoorte as narrow gauge. For the first six months after the line was completed, no locomotives were available, so wagons on the line were towed by horses. It was converted to broad gauge with a new terminus one kilometre east of Kingston, on the edge of the port township in 1957. It closed on 28 November 1987.
Another narrow gauge railway was built from the port on Rivoli Bay at what is now Beachport inland via Millicent to Mount Gambier in 1878. The line and jetty at Beachport provided the ability for farms in the district to export wool and grain. When the line was converted to broad gauge in 1957, it was cut back and no longer served Beachport, but only Mount Gambier to Millicent until it closed in April 1995.
The railway connecting Mount Gambier to Naracoorte was initially approved by the Parliament of South Australia in 1867 to be built to gauge. However it was not built at this time, and that act was repealed by a later authorisation in 1884 to build it on the same alignment to gauge.
On 21 September 1881, the first section of the line opened from Naracoorte to Tatiara. It was extended north to Wolseley on 18 April 1883 and south from Naracoorte to Mount Gambier on 14 June 1887.
In 2001, expressions of interest were sought for a private operator to reopen the line with the state government offering financial assistance to gauge convert, but nothing came of it. If done so it would be the first in Australia to have all 3 gauges. 
Part of the line was used by the Limestone Coast Railway tourist service, until it ceased 1 July 2006. On 15 August 2018 the SARRA (South Australian Regional Rail Aliance) called on the State Government to standardise the line for freight and possible passenger services but still nothing has come of this either.
On 22 August 1904, a 14.2-kilometre narrow gauge branch-line was constructed from Wandilo, 13 kilometres north of Mount Gambier, to Glencoe. The branch closed in June 1957 when the Mount Gambier Wolseley line was all converted to only broad gauge.
On 28 November 1917, the Victorian Railways opened a broad gauge line from Heywood. This line was closed on 11 April 1995. Part of this line was used for Limestone Coast Railway tourist services to the border until it ceased 20 March 1999.  The Heywood line was permanently severed from the Wolseley line when the remains of Mount Gambier railway station was made into a public community space in 2015.
The stations and sidings included: