In its entirety, this line follows the original 1875 North Island Main Trunk route between central Auckland and Pukekohe.

A future third main line is under construction as part of the Wiri to Quay Park project announced in 2017; work started in 2020 and is to be completed by 2025.

[1] It is expected to ease congestion on Auckland rail lines by allowing "through" trains to pass stationary trains at stations, improve rail freight access from the Port of Auckland to the Westfield yards and allow more frequent passenger and freight services.

Along with the rest of Auckland's suburban railway network, the Southern Line was electrified from Waitematā to Papakura, with electric services beginning in 2014.

In 2020, as part of the New Zealand Upgrade Programme, the government announced $371 million in funding to extend electrification from Papakura to Pukekohe.