• No. 171
  
• SECR Plain Green
  
• Dome-riveted smokebox
  
• Modified tender with rear steps and small toolbox
  
• Coreless motor and flywheel
  
• 21 Pin decoder socket in tender
  
• Plunger loco pickups
  
• Tender wheel bearing pickups
  
• Firebox glow
  
• Twin speakers in tender
  
• NEM pockets front and rear
  
• Cosmetic scale coupling
  
• Etched number plates in detail bag
  
• Route indicator discs in detail bag
  
• UK-Designed

In 1878, James Stirling was appointed as the Chief Locomotive Superintendent of the South Eastern Railway. He quickly determined an urgent need for new and more powerful goods and passenger locos.

Bringing his experience from the Glasgow and South Western Railway, he proposed some new designs that were distinctly similar to those he had used for the Scottish Company. One such design was a 0-6-0 tender goods locomotive. Stirling's designs were disputed by Ashford Works Manager, Richard Mansell, who had suggested incorporating tried and tested SER features. Stirling insisted that his design be trusted, and his design was backed by the railway's Board of Directors.

 
Tenders for the contract were received by several locomotive builders, and Sharp, Stewart and Co. were awarded the contract to build the first 20 of the class. This new loco would be designated as the O Class.

 
The SER O Class had a domeless boiler with a Ramsbottom safety valve, a Stirling steam reverser, and a round-topped cab design, which was characteristic of other Stirling designs. It had a stylish tender with springs that protruded above the running plate and alongside the tender sides.

 
A total of 122 of the class were built between 1878 and 1899, with versions being produced almost every year. Sharp, Stewart and Co built a total of 65, and the rest were built at the railway's Ashford Works. Upon the merger of the SER and LC&DR into the SECR, the final 5 examples were built under the new joint committee’s ownership.
The earliest examples that were built by Sharp, Stewart and Co sadly suffered from poor steaming and weak frames, proving Richard Mansell’s fears were correct. This led to Ashford making modifications to the boiler and structure. The new standards were incorporated into future versions.

 
Between 1900 and 1923, 28 examples of the O class were given replacement boilers. These had deeper fireboxes, meaning they were mounted higher on the loco’s frames. But they were otherwise of similar dimensions to the originals.

 
The O Class were used for loose coupled heavy goods trains in their early years, until 1886 when some were fitted with vacuum brakes, and 2 received Westinghouse brakes. O Classes were known for hauling both passengers and goods, but were predominantly a goods engine. They could be seen on routes across the southeast, and even travelled as far west as Reading on regular services.

 
Unfortunately, like many locos of this era, they were superseded by more powerful and efficient classes that were of a similar size. The SECR C Class was the O Class's replacement on the heavy freight and passenger services, relegating the O to lighter duties, shunting or branchline work.

 
In 1904, Harry Wainwright, who was the new Locomotive, Carriage and Wagon Superintendent of the SECR, set about improving the performance of the O Class. He did this by refitting many of them with his latest domed boiler design and new cab. This rebuilt class would be designated as the O1. Conversions continued through to 1932, by which point 59 of the O Class had been rebuilt into O1s.

 
During WW1, the O Class were often used in pairs or as a pilot for the O1 or C Class. Some were even fitted with slip couplings. The War Office requested the SECR provide 10 0-6-0 light goods locos, but the SECR refused to transfer them unless they received suitable replacements from northern companies. An increase in suburban services during the 1920s did see many of the O class return to passenger hauling services.

 
When the railways were grouped into the Big 4 in 1923, 31 O classes entered Southern Railway ownership, and a handful of examples were also sold on to the East Kent Railway.
A significant number of the class had already been scrapped during their SECR days, and with the C, H, R and refitted O1s taking duties previously hauled by the O Class, their days were numbered. The final O Class was withdrawn from service by the Southern Railway in 1932.