The DD class (later reclassified into D1, D2 and D3 subclasses) was a passenger and mixed traffic steam locomotive that ran on Victorian Railways from 1902 to 1974. Originally introduced on mainline express passenger services, they were quickly superseded by the much larger A2 class and were relegated to secondary and branch line passenger and goods service, where they gave excellent service for the next fifty years. The DD design was adapted into a 4-6-2T tank locomotive for suburban passenger use, the DDE (later D4) class. They were the most numerous locomotive class on the VR, with a total of 261 DD and 58 locomotives built.

The Victorian Railways (VR), trading from 1974 as VicRail, was the state-owned operator of most rail transport in the Australian state of Victoria from 1859 to 1983. The first railways in Victoria were private companies, but when these companies failed or defaulted, the Victorian Railways was established to take over their operations. Most of the lines operated by the Victorian Railways were of 5 ft 3 in. However, the railways also operated up to five 2 ft 6 in narrow gauge lines between 1898 and 1962, and a 4 ft 8+1⁄2 in standard gauge line between Albury and Melbourne from 1961.