In 1912 construction began on the standard gauge Trans-Australian Railway, completing the west-east link across the continent. The newly built route was located in the states of Western Australia and South Australia and ran 1,063 miles from Kalgoorlie to Port Augusta. The Commonwealth Railways were founded to operate this route, which later also took over north-south connections on the Central Australia Railway. 26 class G locomotives were purchased for passenger service and were initially used to build the line. Work stalled somewhat during the First World War, but when it opened in 1917 the units were able to serve their intended purpose as passenger locomotives and also hauled the Trans-Australian Express.
G20 circa 1925 on the Trans-Australian Express at Tarcoola
The locomotives were based on the New South Wales Government Railways class P (later C32) built from 1892 to 1911. They differed from these mainly by having a four-axle tender instead of a three-axle tender to cover longer distances. On the locomotives built by Clyde Engineering, the tenders held five long tons of coal and 3,650 gallons of water. Baldwin and Toowoomba, on the other hand, used tenders carrying seven and a half long tons of coal and 4,500 gallons of water. Later they got an even bigger tender with ten long tons and 4,850 gallons.
Schematic drawing with dimensions
Commonwealth Railways 
In terms of boiler pressure and cylinder dimensions, they corresponded to the most recently built series of their prototypes and they were also initially operated using saturated steam. While the class P locomotives were retrofitted with a superheater as early as 1914, only seven class G units received one in the 1930s. In this form they were used as class Ga and from 1938 they were replaced by the heavier class C locomotives and gradually phased out. As the decommissioning dragged on, eleven class G and Ga locomotives were still available in 1951. However, the introduction of diesel locomotives that started now accelerated the phase-out, so that by the following year almost all of them had disappeared and only a single engine served as a shunting locomotive until 1958.