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The mainline goods locomotives later classified as O class ran on the Victorian Railways between 1862 and 1922. Sharing many design features in common with the 2-4-0 passenger locomotives ordered at the same time (later classified as the B class), these goods locomotives were built for hauling traffic over the steeply graded new main lines to Ballarat and the Murray River and were significantly larger and more powerful than their predecessors. Despite a number of accidents including four boiler explosions, the class was highly regarded and its career on the Victorian Railways spanned some sixty years.

History
The recently formed Victorian Railways, having purchased the struggling Melbourne, Mount Alexander and Murray River Railway Company in 1856 and more recently the Geelong and Melbourne Railway Company's line to Geelong in 1860, had begun the construction of two new mainlines to connect the booming gold-mining towns of Ballarat, Castlemaine and Sandhurst, as well as tap the lucrative Murray River trade at Echuca. These ambitious new railway projects, despite being engineered to very high (and very expensive) standards, traversed some difficult terrain and featured numerous gradients of up to 1 in 50. The 2-2-2 passenger locomotives VR had used to operate its relatively flat Geelong line proved to be particularly unsuitable for heavy grades, and VR's fleet was already stretched to service the needs of its rapidly expanding network.

New locomotives were ordered of both a 2-4-0 passenger and 0-6-0 goods type, with many shared design features, with an initial order for six goods engines placed with Slaughter, Grüning and Company.

Production
Williamstown-built O 129 was displayed at the Melbourne International Exhibition in 1880.

Service life
The O class remained the most effective and numerous VR goods lomotive for over 20 years, even after more modern (yet arguably less successful) locomotives had joined the fleet. It was not until the introduction of the Victorian Railways Y class main line goods locomotives of 1889 that the O class was superseded, but even so the class remained at main line depots, with the VR 1894 Rolling Stock Branch Diagram book noting their allocation as 10 at Melbourne, 14 at Bendigo, and ten each at Ballarat and Benalla. The entire class lasted well into the Twentieth Century, and in their later life some were used for shunting operations, with O 25, 27, 35 and 45 fitted with shunting boards for this purpose from 1904 to 1907.

Design improvements
As with the B class, the O class locomotives were reboilered with larger, higher pressure boilers, with their pressure increasing to 140 psi. The boiler pressure was later increased in 1907 to 160 psi Together with an increase in cylinder bore from 17 in to 17+1/2 in, these changes saw tractive effort increase from 11560 lbf to 15680 lbf.

Accidents
For a class of 44 locomotives, the O class was noted as being somewhat accident-prone with no fewer than 20 major incidents being recorded. The original boilers were fitted with Salter-type safety valves which enginemen could adjust by hanging weights on the levers to increase or decrease working pressure, leading to boiler explosions. The class suffered four boiler explosions and O 51 actually exploded twice, at North Melbourne in 1873 and again at Elphinstone in 1887.

Demise
The first O class locomotives withdrawn wer O 43 and O 47 on 13 August 1904, each by this stage having provided over 40 years of service. Another six were taken off the register during 1905, and another four in 1906. Between December 1908 and May 1909, Os 31, 35, 39, 41, 53, 61 and 63 were taken off the register and converted into stationary engine use at Newport Workshops. Much of the remainder of the class remained in service until late in the following decade, and by 1920 just three were left. The last in service, O 135, was written off on 10 June 1922.