The WAGR Dd Class was a class of tank engine steam locomotive used by the Western Australian Government Railways

Western Australian Government Railways

Western Australian Government Railways was most common name of the Western Australian government rail transport authority from 1890 to 1976. It is, in its current form, known as the Public Transport Authority of Western Australia....

 between 1946 and the end of steam on Western Australia's suburban railway network. The Dd type tank engines were based on the earlier Dm Class

WAGR Dm Class

The WAGR Dm Class was a type of 4-6-4T tank engine used by the Western Australian Government Railways from 1945. It was based on the earlier D class but differed significantly in the addition of large boiler-length side tanks, and was built by converting older E Class tender locomotives...

 which were in turn a reconstruction of the earlier WAGR D Class

WAGR D Class

The WAGR D Class was a type of 4-6-4T tank engine used by the Western Australian Government Railways from 1912. It was used mostly on suburban passenger workings and was the basis for the later Dm and Dd classes of tank engine which continued this role until the end of steam on WA's Railways....

 tank engines, using components of the old E Class

WAGR E Class

The WAGR E class were a type of sixty-five steam-locomotives built for the Western Australian Government Railways narrow-gauge network by three British manufacturers Nasmyth, Wilson and Company, Vulcan Foundry, and the North British Locomotive Company, from 1902...

 tender engines. All of the D-type tank engines featured a 4-6-4

4-6-4

Under the Whyte notation for the classification of steam locomotives, 4-6-4 represents the wheel arrangement of four leading wheels on two axles , six powered and coupled driving wheels on three axles, and four trailing wheels on two axles .Other equivalent classifications are:UIC classification:...

wheel configuration.