Between 1953 and 1955, the South African Railways placed ninety Class 25 condensing steam locomotives with a 4-8-4 Northern type wheel arrangement in service.

[1][2] Owing to the difficulties experienced to obtain adequate supplies of suitable water in arid regions like the Great Karoo between Touws River and Kimberley and from De Aar into South West Africa (SWA), the South African Railways (SAR) began to give serious consideration to the possibility of introducing condensing locomotives as far back as the late 1930s.

Between 1886 and 1888, three well-tank condensing locomotives with a 0-4-0 wheel arrangement were placed in service by the Cape Copper Mining Company on its Namaqualand Railway, a 2 ft 6 in (762 mm) gauge line between Port Nolloth and O'okiep.

[3] On the SAR, it was only after World War II, that extensive condensing tests were carried out with the modified Class 20 locomotive.

The approximately 90% water and 10% coal savings which were achieved during the tests with the Class 20 in the Eastern Transvaal and the Karoo in 1950 and 1951 led to the decision to proceed with the design of a new condensing locomotive.

[1] The result, the Class 25 4-8-4 Northern type condensing loco­motive, can be considered as the ultimate in SAR non-articulated steam loco­motive design.

As delivered, the Class 25 had the usual round smokebox front, but this was later modified by adding a banjo-shaped extension to also cover the exhaust steam turbine.

Spent steam was fed through a thick pipe on the engine's left hand side back to the condensing tender.

[12] Almost one third of the total length of the Type CZ condensing tender was taken up by the coal bunker, which included the oil separator equipment to remove oil from spent steam and the mechanical stoker equipment which had a maximum delivery rate of 12,000 pounds (5,443 kilograms) of coal per hour.

[1][2][6][12] Since the temperature of the condensed feedwater was too high for the use of ordinary injectors, the boiler was fed by two turbo-pumps located under the cab.

[1][2] Soon after being placed in service, problems were experienced with failing connecting rods, big end bearings breaking up as well as cracks developing in the motion girder of the Alligator crossheads.

After investigations by SAR engineers with assistance from South Africa's Council for Scientific and Industrial Research (CSIR), the crossheads, slide bars and coupling rods were modified.

The crossheads were converted to the multiple-bearing type with single guide bars, a more sophisticated method of filtering out the cylinder and valve lubricant from the exhaust steam replaced the original centrifuge while the three independent coupling rods were replaced with the more conventional single coupling rod with knuckle joints.

[1][13][17] When new, the tapered Timken crankpin roller bearings soon became notorious for throwing their lubricant onto the underside of the boiler, from where it ran down to the lowest point and dripped onto the coupled wheel tyres along the way.

The solution was arrived at when it was realised that none of the single "lock" blades, which were supported by two conical pins, of any rotor had ever failed.

When such conical pins were also introduced between all the other blades in the rotors, fatigue tests showed that this made them considerably stronger.

The design was eventually amended with the redesigned exhaust fan being manufactured from manganese steel and the problem was solved.

From Postmasburg they worked iron and manganese ore to Bloemfontein, where relays of Free State power took over to Harrismith.

[11][16][19] The Class 25 was a complex locomotive which required high maintenance, especially on the turbine blower fans in the smokebox, the blades of which needed to be replaced frequently due to damage by solid particles in the exhaust.

The final form of the rebuilt tender's tank supplied enough strength, with its semi-circular top welded to the original fresh water tank via the fan supports and the long triangular gussets set into the bunker sides which extended past the midpoint of the frame.

Locomotives with rebuilt tenders were soon nicknamed Worshond, Afrikaans for dachshund and literally translated as sausage dog.

After they were relieved of their condensing gear, these locomotives served for another eleven years, before being withdrawn from service by the SAR when steam was completely replaced by electric and diesel-electric traction.