The line replaced the original, single-track section of 1867 that proceeded via Thirlmere and Hill Top. As told in the previous series, it was approved by NSW Parliament in December 1914. Although the Great War had begun, it was decided to press on with urgent railway deviation and duplication work.

Two tunnels were built for the deviation to ascend in a wide sweep to the Mittagong plateau, where it connected at Braemar to the main line, completing duplication of the entire Southern Railway to Goulburn.

Train delays became less frequent and journey times shortened somewhat, although for travellers and freight to and from the local district, the improvement was not all that significant. Residents of the district's northern villages, however, were more directly impacted by the deviation - those along the original line lost their advantage whereas those on the new section benefitted.

A new railway time-table for the Southern Line was brought into force. From July 19, 1919, the Melbourne expresses and all mail and through passenger trains ran on the deviation from Picton to Mittagong, via Bargo, including the Cooma mail trains which had been temporarily discontinued between Sydney and Goulburn. On the old section from Picton to Mittagong, via Thirlmere and Hill Top, known as the loop line, a local train service was provided, connecting at Picton with main line trains.

Stations were provided along the new section at Bargo, Yerrinbool and Aylmerton, opening on July 19, 1919, and at Yanderra, opening in 1924. In anticipation of the new line, from 1917 subdivisions were created on land near these stations, thus heralding a boom in real estate.

The deviation opened 100 years ago this month. Centenary celebrations will be held by Wingecarribee Shire's northern residents at Yerrinbool Railway Station and the Community Hall on Saturday, July 13, starting at 10.00am. All those with an interest in the area's history are invited to attend.

The first road to the southern district was a cart track built on Governor Macquarie's orders by convict labour in 1819 with explorer Dr Charles Throsby in charge. Commencing at Picton, it crossed the Bargo River and then proceeded through the thickly wooded Bargo Brush, originally known as Cannabaygal's Plains, named after a local Aboriginal chief. It then entered the area around present-day Yerrinbool and through Chalkerville (present-day Aylmerton locality) to cross the steep Mittagong Range (now Old South Road) and down to Bong Bong at the Wingecarribee River. It then proceeded on to Sutton Forest and Goulburn.

In the early 1830s, Surveyor-General Major Mitchell re-routed a section of the road to avoid the Mittagong Range. His new line wound up Catherine's Hill (at the top today is Alpine) and proceeded through present-day Mittagong to a new crossing of the Wingecarribee River, where Berrima was established, rejoining the original road several miles further on.

Taverns and inns soon opened along the new line of road. In order to reach Nattai, south-bound travellers had first to venture through the dreaded Bargo Brush. One description states that "it was no uncommon thing to see the whole way dotted with vehicles, from the gig and spring cart to the bullock dray, all stuck fast in the numerous sloughs, filled with mud, which looked most treacherously level, deceiving even the wary". It became the haunt of escaped convicts and bushrangers. Once through, travellers came to a comfortable sandstone tavern built by John Keighran in 1831 at the foot of Catherine's Hill.

Travel times on the Great Southern Road improved somewhat in the 1850s with the introduction of Cobb and Co coaches that covered up to 40 miles a day, but through-traffic on the road drastically declined once the Great Southern Railway opened to Goulburn in 1869 and provided safer, more efficient transport.

The road continued in use for local purposes, including as a feeder to Colo Vale Station for fresh produce from the Bargo Brush area, where horticulture had been established.