Victorian Railways class D^D

The class D^D express locomotive marked the change from the 4-4-0 to the 4-6-0 wheel arrangement for the Victorian Railways. It was the railway's first locomotive, which they developed themselves in 1902 and which was no longer based on British designs. As early as 1907 it was superseded by the more powerful class A² and was no longer used in express service.

Thanks to its low axle load, it could now be used in mixed service on branch lines. As early as 1907, 58 were rebuilt into 4-6-2T tank locomotives for suburban service in Melbourne and designated class D^D_E. They were later given the designation D⁴, but were scrapped in 1924 and 1925 due to electrification.

Production of the normal tender variant ran until 1916, with individual production lots being put out to tender again and again. In order to test the production of their indigenous design abroad, 20 were built at Beyer, Peacock & Co. and at Baldwin. The last locomotives were already delivered with superheaters, and many existing ones were retrofitted. Just as the tank variant was soon called D⁴, the saturated locomotives were soon called D¹ and the superheated ones D².

A total of 261 were built. Between 1929 and 1947, 93 locomotives were rebuilt with the more powerful and superheated class K boiler, which now became the D³. About half were scrapped by about 1950, and most of the rest by the 1960s. Only one, used as an inspection locomotive, remained in service until 1974. Today only the 639 is still operational and around 15 others are existing or preserved non-operational.