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.

The DD design was adapted into a 4-6-2T tank locomotive for suburban passenger use, the DDE (later D4) class.

By 1900, Victoria's express passenger locomotive fleet was almost exclusively made up of 4-4-0 designs of the Old A, New A, and the most recent AA class.

[1] At the turn of the century, in what marked a major shift in policy, the recently appointed VR Commissioner, John Mathieson, set up a Locomotive Design Section for in-house development of future motive power.

[2] A 4-6-0 design equipped with 5 ft 1 in driving wheels, saturated steam boiler and Belpaire firebox, the DD reflected the considerable talent of VR's design team, which included ex-Beyer, Peacock & Company recruit Eugene Siepen, future VR Chief Mechanical Engineer Alfred Smith, and Rolling Stock Branch manager Thomas Woodroffe.

By this point the odds/evens locomotive numbering scheme had been abandoned, so the last nine of the batch were delivered as 701-717 to start filling gaps.

As part of the competitive tendering process, in early 1912 contracts were signed with each of Beyer, Peacock & Company of Manchester, England, Baldwin Locomotive Works of the US, Walkers Limited of Maryborough, Queensland and Austral Otis, to compare against the cost of building engines at Newport Workshops.

The firm Thompsons & Co successfully won the contract for the 20 engines not being constructed by Austral Otis, and these were delivered from the end of 1914 numbered 893-912.

A repeat order was placed in 1916 with deliveries of 963-982, and work had started on a further 20 engines when pressures of World War I led to the firm abandoning the remainder of the DD contract extensions.

These three workshops turned out virtually all subsequent locomotives for the Victorian railway system until the post-war era.

[5]) DD class locomotives were initially assigned to hauling the Adelaide Express over the steep gradients between Melbourne and Ballarat, but were soon seen on mainline passenger services on a number of lines.

[6] The first years of the 20th century saw on the VR (as elsewhere in the world) a considerable increase in both the amount of traffic and the size and weight of rolling stock being hauled.

[7] However, with their light axle load (just 12 t 10 cwt in their original form),[8] they were quickly reassigned to the VR's branchline network, where they became a fixture for the next fifty years.

[9] From July until September 1918, 1032 was loaned to the South Australian Railways for trails against a Rx class operating from Adelaide to Murray Bridge and Victor Harbor.

[10] With their light axle load and express passenger speed, the DD was also an ideal choice as motive power for the Victorian Railways Commissioner's train (used to carry the VR Commissioners on inspection tours to every corner of the VR network).

The new Commissioners engine from 1937 was D3 683, specially fitted with an electric headlight (Mort Clark Bulletin Article) and in August 1950 it was replaced by D3 639.

In 1983 new Chief General Manager Mr. John Hearsch reinstated the Inspection Train with Clyde diesel-electric T 410.

They were put to work on longer and hillier suburban routes such as the Dandenong, Frankston, Upper Ferntree Gully, Williamstown, Werribee, Lilydale, Darling and Kew railway lines.

was designed for easy conversion to DD tender engines in the event of electrification making them redundant.

The first locomotives built featured low running plates with splashers over the driving wheels and a narrow cab.

However, after 26 such examples were built the design was altered[13] with high running plates mounted above the driving wheels and a more comfortable full-width pressed metal cab of Canadian design, a feature incorporated at the request of Victorian Railways Chief Commissioner and former Canadian Pacific Transportation Manager Thomas Tait.

Although the Dd was considered to be a successful design, it had a key shortcoming in that its boiler performance was not sufficient for the traffic demands being placed on it.

[13] In 1923–4, DD 1022 was experimentally fitted with Pulverised Brown Coal (PBC) burning equipment.

In 1922, a new design of 2-8-0 branch line goods locomotive, the K class, was introduced, with noticeably superior boiler performance to that of the DD.

A full 20 engines (including the newest of the fleet, DD 1052) were scrapped in 1929 as newer K and N class locomotives took over branch line goods services and Petrol Electric Rail Motors started to replace mixed trains and locomotive-hauled branch line passenger services.

D3 639 was restored to operating condition in 1984 and was recommissioned into service by Prime Minister Bob Hawke on 17 November 1984.

[24] From 5 December 1970 the engine was painted red with black undergear and a brass dome,[25] and by the Austeam '88 festival it had been named "Spirit of Ballarat".

13 other D3 class locomotives remain, either preserved in static display or stored awaiting restoration or as a supply of parts.