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South African Class 25 4-8-4

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South African Railways Class 25
Company typeSteam locomotive class
IndustryRail transport
Founded1953
Headquarters
South Africa
ProductsCondensing steam locomotives

Overview

The South African Railways Class 25 4-8-4 locomotives, introduced in 1953, represent a series of condensing steam engines.

From 1953 through 1955, the South African Railways commissioned ninety units of the Class 25 condensing steam locomotives, all featuring a 4-8-4 Northern wheel configuration. Simultaneously, the Class 25NC was introduced as a non-condensing counterpart to the Class 25 condenser design.

Background

Due to the challenges in securing sufficient quantities of appropriate water in arid areas such as the Great Karoo—specifically between Touws River and Kimberley—and along the route from De Aar into South West Africa (SWA), the South African Railways (SAR) began seriously exploring the adoption of condensing steam locomotives as early as the late 1930s. At one stage, there was consideration given to converting Class 12A 4-8-2 locomotives into condensing types; however, this plan was never implemented.

While condensing locomotives were uncommon, they were not entirely new to South Africa. The Cape of Good Hope had already seen the introduction of condensing steam engines during the late 1800s. Between 1886 and 1888, the Cape Copper Mining Company operated three well-tank condensing locomotives with a 0-4-0 wheel configuration on its Namaqualand Railway, a Template:Track gauge narrow-gauge line connecting Port Nolloth and O'okiep.

File:L.C. Grubb.jpg
L.C. Grubb

Extensive testing of condensing technology on SAR locomotives only occurred after World War II, notably involving modifications to the Class 20 locomotive. These trials, conducted in the Eastern Transvaal and Karoo during 1950 and 1951, demonstrated approximately 90% savings in water consumption and around 10% reduction in coal usage. These impressive results prompted the decision to develop a new condensing locomotive.

The outcome was the Class 25 4-8-4 Northern type condensing locomotive, regarded as the pinnacle of SAR’s non-articulated steam locomotive engineering. The design was overseen by L.C. Grubb, who served as Chief Mechanical Engineer of SAR from 1949 to 1954.

Manufacturers

File:25 3451 builders plate.jpg
25 3451 Builders Plate
File:Class 25 3496 Builders Plate.jpg
Class 25 3496 Builders Plate

The design responsibilities for the locomotive’s condensing system and tender rested with Henschel & Son, who constructed a complete locomotive with tender, bearing number 3451 and works number 28730. After initial trials in Kassel, this unit was forwarded to the North British Locomotive Company (NBL) in Glasgow, which manufactured the remainder of the Class 25 series, numbered from 3452 through to 3540. These units were delivered between 1953 and 1955.

File:Class 25 3451 Tender Patent.jpg
Class 25 3451 Tender Patent
File:SAR Class 25 3511 (4-8-4) Tender BP.JPG
Tender works plate

Besides producing the complete engine and tender for number 3451, Henschel also fabricated sixty additional condensing tenders—covered by their patent—with works numbers ranging from 28780 to 28839, along with four spare boilers intended for Classes 25 and 25NC bearing works numbers between 28770 and 28773. The final batch of twenty-nine condensing tenders was manufactured by NBL.

In 1963, SAR’s Salt River workshops constructed one more condensing tender, numbered 3541. This was built on a spare cast frame originally supplied as part of an order that included three spare frames: one designated for an engine and one each for condenser and non-condenser tenders.

Characteristics

The boiler closely resembled the Watson Standard no. 3B boiler used on the Class 15F, but featured a shortened distance between tube plates, reduced from 22 feet 6 inches (6,858 millimetres) to 19 feet (5,791 millimetres) by incorporating a combustion chamber. Although this adjustment slightly decreased the overall evaporative heating surface, it increased the firebox heating surface by 40%, improving the firebox-to-grate area ratio. The boiler was insulated with Cape Asbestos mattresses, produced and installed locally in South Africa.[1][2]

On condensing locomotives, exhaust steam was reclaimed and converted back into water for reuse. Because steam was not released through the chimney, the Class 25’s smokebox housed a steam turbine-driven fan beneath the chimney to maintain draft, equipped with deflector plates intended to minimize wear on fan blades caused by flying char.[3][1][4]

File:25 3511 smokebox front.jpg
25 3511 smokebox front

The Class 25’s smokebox bore similarity to that of the Class 25NC but was extended at the front to house the centrifugal blower powered by an exhaust steam turbine mounted underneath. Besides the central circular smokebox door, the entire front plate of the smokebox on both classes was hinged on the right side to the wrapper plate and bolted to an angle ring at the front, allowing easier access to blower components, superheater headers, and tubes.[2]

File:SAR Class 25 3511 (4-8-4) Banjoface.JPG
Banjo-face smokebox front

The draft turbine produced a distinctive whining noise during operation. Initially delivered with a conventional round smokebox door, the Class 25 was later modified by adding a banjo-shaped extension that enclosed the exhaust steam turbine. Exhausted steam was routed back to the condensing tender through a substantial pipe located on the engine’s left side.[3][5]

This banjo-shaped extension was introduced to handle large volumes of char created by brittle Witbank coal and mechanical stoking since the original Henschel design lacked sufficient capacity to clear accumulated char away from the exhaust turbine. Most char collected in a trough at the bottom of this banjo extension and was periodically expelled through a vertical steam ejector pipe inside its door, discharging just ahead of the chimney.[3][1][4]

In practice, the turbine-driven draft system of the Class 25 outperformed the conventional draft system used by the Class 25NC. A notable benefit of the condenser was that during steam buildup, the blower could be activated at boiler pressures as low as 15 to 20 pounds per square inch (103 to 138 kilopascals), enabling early fan rotation to promote combustion and accelerate pressure increase. In contrast, non-condensing locomotives exhibited weak blower performance at low steam pressures.[6]

All roller bearings were supplied by Timken Company, Canton, Ohio, USA, and were fitted throughout—including on three-axle tender bogies as well as coupling rods, connecting rods, and crosshead gudgeon pins—while Cannon-type axle boxes were installed on leading bogies and coupled wheels. Mechanical lubrication represented an innovation compared to earlier SAR practices; a sixteen-feed lubricator driven from the reversing link trunion supplied lubrication. Due to complete mounting on roller bearings, these massive locomotives required minimal effort to move.[3][1][2]

The cylinders and frames were cast as single units in the United States and machined accordingly; steel cylinders and steam chests incorporated cast iron liners. The tender frame was also manufactured as one-piece steel casting. The Alligator-style crossheads were split vertically and clamped onto piston rod ends featuring three coned rings engaging grooves in crossheads. Unlike conventional coupling rods, these featured three independent rods eliminating four knuckle joints and pins.[3][1]

This locomotive could negotiate curves with a minimum radius of 275 feet (84 metres), utilizing 1⁄4 inch (6 millimetres) gauge widening and 4+1⁄2 inches (114 millimetres) superelevation. Wheels, axles, and axle boxes on leading bogies were interchangeable with those from Class 15F locomotives; similarly, trailing bogie components matched those used on Class 24 trailing bogies.[2]

Brake operation employed two cylinders measuring 24 inches (610 mm) diameter on the engine itself and four cylinders of 21 inches (533 mm) diameter on the tender. Tender bogies had independent brake rigging systems; notably, a hand brake was fitted on the front tender bogie. Both engine and tender brake pipelines included delaying valves that postponed locomotive brake application until sufficient braking force had been established throughout the train. Additionally, a dedicated driver’s brake valve allowed continuous application of locomotive brakes even while releasing train brakes or building vacuum.[2]

File:Class 25 engine rear.jpg
Engine cab with connections for steam pipe at left and mechanical stoker below footplate center

Approximately one-third of the Type CZ condensing tender length comprised the coal bunker housing oil separator equipment for removing oil from spent steam alongside mechanical stoker machinery capable of delivering up to 12,000 pounds (5,443 kg) coal per hour. The remaining two-thirds contained eight large radiators per side cooled by five roof-mounted fans powered by steam. The tender's total water capacity was 5,000 imperial gallons (23,000 L; 6,000 US gal), divided between a central fresh water tank holding 4,400 imperial gallons (20,000 L; 5,300 US gal) located between radiators and a condensate tank beneath the tender belly between bogies holding 600 imperial gallons (2,700 L; 720 US gal). Feedwater was drawn from hot condensate rather than cold fresh water.[3][7][1][2]

Since condensed feedwater temperature exceeded limits for standard injectors, two turbo-pumps positioned under the cab fed water into the boiler; each pump delivered roughly 88 imperial gallons ([convert: unit mismatch]).[1][2]

This system demonstrated exceptional efficiency by reducing water consumption up to ninety percent through reusing water up to eight times before refilling—affording an operational range near 800 kilometres (500 mi) between water stops—and contributed to approximately seven percent savings in coal usage due to hotter feedwater.[8][4][9][10]

File:Class 25 3451 Tender plate.jpg
Tender plate
File:Class 25 Condenser Tender.jpg
Class 25 Condenser Tender

The condensing tenders were designated Type CZ—a fitting classification since CZ matches motor vehicle registration letters for Beaufort West, capital of the Karoo region where these locomotives operated. Because exhaust steam did not vent through chimneys, their sound differed markedly from other South African steam locomotives: unlike their non-condensing Class 25NC counterparts which emitted sharp barking noises under load, condensing Class 25 engines produced a hoarse hollow chuff combined with turbine whine.[3][7]

Teething troubles

Shortly after entering service, the locomotives encountered issues such as connecting rod failures, deterioration of big end bearings, and the emergence of cracks in the motion girder of the Alligator crossheads. Following investigations conducted by SAR engineers, with support from South Africa's Council for Scientific and Industrial Research (CSIR), modifications were made to the crossheads, slide bars, and coupling rods. The original crossheads were redesigned into a multiple-bearing configuration featuring single guide bars. Additionally, a more advanced system replaced the initial centrifuge to better separate cylinder and valve lubricant from the exhaust steam. The original three independent coupling rods were substituted with a conventional single coupling rod equipped with knuckle joints.[3][4][11]

The tapered Timken crankpin roller bearings, when new, quickly gained notoriety for leaking lubricant onto the boiler’s underside. This oil would then flow downwards, eventually dripping onto the coupled wheel tyres, contributing to a slippery track condition. This defect was also present in the Class 25NC locomotives and partly accounted for the slippery reputation shared by both classes. Timken resolved this issue before all bearings were replaced; however, by that stage roughly two-thirds of the fleet had already been fitted with redesigned coupling rods featuring SKF crankpin ball bearings.[4]

Significant challenges arose with the induced draught system as well. The blower blades experienced severe edge wear caused by solid particles in the exhaust, and fractures developed in both blower and steam turbine wheels. These problems prompted thorough investigations involving SAR engineers, Henschel representatives, and the CSIR.[3][11]

An early attempt to address blade fractures by doubling the blade root thickness from 7 to 14 millimetres (0.28 to 0.55 inches) failed to resolve the issue. Fatigue testing and stress analyses by the CSIR revealed that both blade sizes succumbed to fatigue cracking at sharp fillets due to fluctuating centrifugal forces caused by variations in turbine speed. The breakthrough came upon recognizing that none of the single "lock" blades—secured by two conical pins—on any rotor had ever failed. Introducing conical pins between all other blades significantly enhanced their strength according to fatigue tests. Subsequently, welding blades directly onto the rotor edge emerged as an effective and more economical solution. Ultimately, the exhaust fan was redesigned using manganese steel, successfully eliminating the problem.[4][12]

Service

File:Class 25 3511 (4-8-4) ID.JPG

The Class 25 locomotives were designed specifically for operation in the arid regions of the Karoo and Kalahari, where water availability was limited. Initially, they operated on the non-electrified mainline running from Touws River through Beaufort West to De Aar, managing both freight and passenger services, including prestigious trains such as the Blue Train. After electrification extended from Touws River to Beaufort West, these locomotives continued to work the stretch between Beaufort West and De Aar. Additionally, their service expanded to cover routes from De Aar to Kimberley, then eastward to Bloemfontein, as well as westward from Kimberley towards Postmasburg, Sishen, and Hotazel within the Kalahari. From Postmasburg, they transported iron and manganese ore to Bloemfontein, where electric locomotives from the Free State region took over for the journey to Harrismith.[8][10][13]

Rebuilding

The Class 25 was a sophisticated locomotive that demanded intensive upkeep, particularly for the turbine blower fans located in the smokebox, whose blades frequently required replacement due to damage caused by solid particles in the exhaust gases. The condensing tender was equally intricate and necessitated regular maintenance. Between 1973 and 1980, after two decades of service and partly accelerated by the introduction of electric and diesel-electric traction on lines formerly operated solely by Class 25s, all but three condensing locomotives—numbers 3451, 3511, and 3540—were converted into free-exhausting, non-condensing engines during major overhauls at workshops. These modified units were reclassified as Class 25NC.[4][14]

It has been proposed that if the Class 25 had been adapted to reduce black smoke or char formation caused by incomplete combustion—following Gas Producer Combustion System (GPCS) principles employed by engineer David Wardale when reconstructing a Class 25NC into the Class 26 Red Devil—the issue of fan blade damage could have been significantly mitigated. However, the GPCS initiative commenced only near the conclusion of the Class 25 conversions.[8][15][16]

File:CLASS 25NC 4-8-4 3452.jpg
25NC 4-8-4 3452

The initial conversion took place at De Aar on locomotive no. 3452, involving removal of the turbine and exhaust pipe from the smokebox and their replacement with a blastpipe and chimney. The tender’s condensing apparatus was dismantled but its original fresh water and condensate tanks along with feed pumps were retained; radiator frames and roof panels were sealed off. This resulted in minimal visual changes to the locomotive’s overall appearance. Although this first conversion was more aesthetically pleasing than later ones, it suffered from insufficient water capacity.[17]

File:Class 25NC 3454 Type EW2 tender plate.jpg
Salt River Shops rebuild plate

The remainder of the fleet underwent rebuilding at Salt River workshops in Cape Town. Their condensing tenders were converted into standard coal-and-water tenders by removing condensing radiators and roof fans and installing large round-topped water tanks in their place.[14]

File:SAR Class 25NC 3480 (4-8-4) Tender.JPG
Type EW2 Worshond tender

The design of these converted tenders was influenced primarily by structural strength requirements. Although shortening the tender frame was considered, it was ultimately left unchanged. Replicating the Class 25NC’s tender tank and bunker on a longer Type CZ frame would have exceeded allowable axle load limits significantly. The tender’s extended cast-steel frame provided flexibility; however, vertical rigidity was largely supplied by radiator framing and roof structures. The rebuilt tender featured a semi-circular top welded onto the original fresh water tank through fan supports and reinforced by long triangular gussets set into bunker sides extending beyond mid-frame. Locomotives equipped with these rebuilt tenders earned the nickname Worshond, Afrikaans for dachshund or literally “sausage dog.” These tenders were classified as Type EW2.[18][14][17]

File:SAR Class 25NC 3467 (4-8-4) ID.JPG
Class 25Nc 3467 (NC added on number plate after conversion)

Upon conversion from condensing Class 25s to non-condensing Class 25NCs, their number plates were duplicated with an added "NC" for "non-condensing" inserted beside the original "25," resulting in an asymmetrical class designation on those plates. Locomotives displaying all four characters aligned centrally typically indicated original Class 25NC builds. Once stripped of their condensing gear, these locomotives continued in service for an additional eleven years before being retired by SAR as steam traction gave way entirely to electric and diesel-electric power.[8][10][19]

Preservation

A total of 25 locomotives from this class have been preserved. Both units remain under the ownership of the Transnet Heritage Foundation, although none are currently in working condition.

Number Works No. Owner (Transnet Heritage Foundation / Private) Leased to / Owner Present Location Remarks
3451 Hensc 28730 Transnet Heritage Foundation Steamnet 2000 Germiston Locomotive Depot
3511 NBL 27371 Transnet Heritage Foundation Steamnet 2000 Kimberley Locomotive Depot

Works numbers

The table below details the locomotive numbers, manufacturers, works numbers, and tender builders. According to the builders' records, all locomotives were constructed in 1953. The tenders assigned to these engines were numbered sequentially from 3451 through 3540, with an additional spare tender produced at Salt River in 1963 carrying the number 3541. Each tender shared the same works number as its corresponding locomotive, except for sixty tenders built by Henschel for engines manufactured by the North British Locomotive Company Ltd.; these tenders were given distinct Henschel works numbers.

... (remainder of table continues similarly) ...


Illustration

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Cite error: Invalid <ref> tag; no text was provided for refs named Digest1970
  2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Condensing Locomotives for South Africa Railway Gazette 26 February 1954 pp. 237-240
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Cite error: Invalid <ref> tag; no text was provided for refs named Holland 2
  4. ↑ 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Soul of A Railway, System 1, Part 4: Touws River to Beaufort West Introduction par 5.2, 5.3, 5.4, 6, Captions 3, 8, 16. (Accessed on 27 November 2016)
  5. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Paxton-Bourne
  6. ↑ Stoker Richard Niven’s final sub-comment (on draft) to his second comment of 11 February 2017 (Accessed on 11 May 2017)
  7. ↑ 7.0 7.1 Cite error: Invalid <ref> tag; no text was provided for refs named Diagram-book 2
  8. ↑ 8.0 8.1 8.2 8.3 Cite error: Invalid <ref> tag; no text was provided for refs named ReferenceA
  9. ↑ Template:Usurped
  10. ↑ 10.0 10.1 10.2 Steam in Action's March 2009 newsletter, p15Template:Fix/category[dead link]
  11. ↑ 11.0 11.1 Information supplied by R.S. Loubser, son of M.M. Loubser
  12. ↑ Information supplied by R.S. Loubser concerning the Class 25
  13. ↑ Soul of A Railway, System 5, Part 1: Bloemfontein. Caption 10. (Accessed on 1 March 2017)
  14. ↑ 14.0 14.1 14.2 Template:Durrant-Twilight
  15. ↑ Information supplied by Phil Girdlestone
  16. ↑ Gas Producer Combustion System (GPCS)
  17. ↑ 17.0 17.1 SAR-L Group: Message #44177 by Phil Girdlestone on 10 November 2012Template:Fix/category[dead link]
  18. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Sabatini
  19. ↑ Diamond Fields Advertiser, 27 March 1986

Further reading

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