The single-track Great Southern Railway opened from Picton to Mittagong in March 1867, and to Goulburn in May 1869.

Locomotives bound for Sydney from southern areas had a relatively easy run except beyond Mittagong, where trains had to be hauled up to Colo Vale and then up to Hill Top before reaching Picton. Even with refuge sidings, by the early 1900s the section became a bottleneck as traffic volumes increased. Delays meant wastage of coal and water.

Calls had begun in the 1890s for the entire Southern Line to be duplicated but, being an expensive undertaking, 20 years passed before work commenced. Duplication of the line between Bowral and Goulburn started in 1913 and, the terrain being relatively level, was completed in 1915.

It was decided to entirely replace the Picton-Mittagong section with a double-line deviation via Bargo, Tahmoor, Yerrinbool and Aylmerton.

Residents along the original route feared that a complete closure would cause them inconvenience and loss. They took their case to the Railway Commissioners, proposing that just a new, single ‘down’ track be built with the original line retained as the ‘up’ line (to Sydney). A compromise was reached: the double track via Bargo would proceed with the old single track retained in local service as the Picton-Mittagong ‘Loop Line’.

The deviation via Bargo required major construction work, including a viaduct at Bargo River and the building of four tunnels. It opened in July 1919.

The loop line connected with the new main section at Braemar and, until the 1980s, a weekly through-service and daily rail motor operated. When derailments or landslides blocked the main line, trains were diverted over the loop line.

At Mittagong Station, major track relocation work and new structures were required for line duplication. A signal box, small timber waiting room and footbridge were built by 1919 for a new down platform (Platform 2) on the duplicated line. Mittagong also served as a junction and change station for the loop line.

In 1967 the Centenary of Mittagong Station was celebrated. The Hon TL Lewis, NSW Minister for Lands and Mines, unveiled a plaque mounted in a block of trachyte stone at the station. Representatives of both the Royal Australian and Berrima District Historical Societies were present, signifying the occasion’s importance.

The Berrima District Post of March 7, 1967 said: “For a moment it seemed the clock had been turned back one hundred years when the vintage steam train pulled into the station with over one hundred passengers dressed in gay 19th century costumes. Every vantage point was taken as five hundred people crowded onto the platform awaiting the arrival of the train. The Police Pipe Band played in the square. It was the biggest day ever for Mittagong.”

Mr T Boyle, representing the Commissioner for Railways, gave an address, stating that the construction of the line from Sydney to Goulburn reached Liverpool in 1856. By 1863 it had reached Picton and by 1867, Mittagong. Cobb & Co coaches ran a service to Goulburn from Mittagong. “In those days the trip to Mittagong took four hours. Today the speedy passenger trains cover the distance to Sydney in just under two hours”.

Representatives from Wingecarribee Shire and Bowral Municipality were welcomed by Councillor G Elliott, President of Mittagong Shire Council who stated that “one hundred years ago this week the rail link from Sydney to Mittagong was completed. It was mainly from the growth of the railway to Mittagong that industry in the area has grown so”.

The year 1967 also heralded the end of the steam era, as most services were converted to diesel by 1969.

Today the Mittagong railway precinct, one of the original major stations along the Great Southern Railway, is an important regional complex and of state heritage significance. Its fine Victorian Georgian railway buildings remain substantially intact.