South African Class 21 2-10-4

The South African Class 21 2-10-4 of 1937 is a South African steam locomotive from the South African Railways era.

In 1937 the South African Railways placed a single Class 21 steam locomotive with a 2-10-4 Texas type wheel arrangement in service, designed as a mixed traffic locomotive suitable for use on light rail. A simultaneously proposed heavier mainline version Class 22 2-10-4 never materialised.^[1][3][4]

Manufacturer

The Class 21 2-10-4 Texas type locomotive was designed by A.G. Watson, Chief Mechanical Engineer (CME) of the South African Railways (SAR) from 1929 to 1936. It was built by the North British Locomotive Company (NBL) in Glasgow and delivered in 1937. Only the one locomotive was built, numbered 2551. At the time the design represented the maximum power obtainable from a ten-coupled non-articulated locomotive that was limited to a 15 long tons (15.2 tonnes) axle load on 60 pounds per yard (30 kilograms per metre) rail.^[1][4]

Design

Engine

Watson disliked articulated locomotives and his aim was to develop a powerful non articulated mixed traffic branchline locomotive with an axle load suitable for light rail. To enable it to negotiate tight curves, the third and fourth coupled wheelsets were flangeless. It used a Watson Standard no. 3B boiler, the same as that used in the Class 15E, Class 15F and Class 23.^[1][4]

The end result could be considered as Watson’s answer to the Class GF 4-6-2+2-6-4 Garratt locomotive, having very similar axle loads. The Class 21 carried more water than the Garratt and was about 23 long tons (23.4 tonnes) heavier with 43,700 pounds-force (194.4 kilonewtons) tractive effort, compared to the 34,200 pounds-force (152.1 kilonewtons) of the Class GF. Even so, only one Class 21 was produced and the design was not repeated.^[1][4][5]

Watson’s design called for cylinders with Rotary Cam Poppet valve gear, but since the locomotive was still under construction when Watson retired, his successor as CME, W.A.J. Day, made use of the opportunity to alter the specifications. Thus, in the year following Watson’s departure, the Class 21 locomotive was delivered with Walschaerts valve gear.^[1][4][5]

Tender

The tender was an unusual experimental type using six pairs of wheels in a 2-8-2 wheel arrangement with the leading and trailing wheels in bissel type pony trucks and the rest of the axles mounted with a rigid wheelbase. The tender’s wheel arrangement did not prove to be very successful and, with the exception of a similar tender built in the Salt River shops for test purposes, was not used again on the SAR.^[1][4]

The pony axle design is commonly used on steam locomotives with a single leading or trailing axle, and was also used on the Class 4E electric and Class 32-000 and 32-200 diesel-electric locomotives.^[4]

The aborted Class 22

A design for a Class 22 steam locomotive, Watson’s final design, was submitted at about the same time. The proposed Class 22 was also to have a 2-10-4 Texas wheel arrangement, but was to be a heavy mainline version of the Class 21, with an axle load of 22 long tons (22.4 tonnes), the heaviest that current SAR track could bear on its 96 pounds per yard (48 kilograms per metre) mainline rail. It was to have been a massive machine with larger 60 inches (1,520 millimetres) coupled wheels, a larger 80 square feet (7.432 square metres) grate and the larger Type EW tender that was later to be used with the Class 23 locomotive.^[1][4][5]

If this locomotive had been built, it may have been one of the world’s most outstanding locomotives. The proposed boiler pressure was 250 pounds per square inch (1,720 kilopascals), a figure never attained on the SAR, and its anticipated tractive effort of 66,406 pounds-force (295.4 kilonewtons) at 75% of boiler pressure would have made it capable of handling loads of 2,200 long tons (2,235 tonnes) on the coal run from Witbank to Johannesburg with comparative ease.^[1][5]

The design was a compromise between a 2-8-4 passenger class with 66 inches (1,680 millimetres) coupled wheels and a 2-10-2 freight locomotive with 60 inches (1,520 millimetres) coupled wheels. At the time, however, the demand for general utility locomotive types was so pronounced that no good argument could be put forward for the introduction of a heavy locomotive dedicated to goods working only. Another factor which acted against the project was the insufficient length of the receiving sidings in the yards, which made it doubtful that such a locomotive would have been able to be used to its full capacity.^[5]

Although the Class 22 was never built, the class number was not used for another steam locomotive type.^[5]

Service

Class 21 number 2551 was mainly used on the line from Pretoria to the Eastern Transvaal and was scrapped in 1952 after only 15 years in service. During its time in service it was equipped with smoke deflectors. Neither the Class 21 nor the aborted Class 22 design was subsequently repeated in either its original or modified form, leaving the impression that they represented advanced thinking that appears to have been considered as too far and too fast by Watson’s successors.^[4]

During World War II this locomotive was often used to haul long and heavy military trains and it was made the official mascot of the then oldest military unit in Pretoria, the Pretoria Regiment (Princess Alice's Own). The Class 21 was the only SAR locomotive to be honoured in this way by the military.^[6]

Sides illustrated

The main picture shows no. 2551 with smoke deflectors installed, circa 1945, while the following photographs show both sides of the locomotive in the factory gray livery as delivered.

See also

• List of South African locomotive classes
• South African locomotive history
• Tender locomotive numbering and classification
• The 2-10-4 "Texas"
• Watson Standard boilers

References

Wikimedia Commons has media related to: South African Class 21 (2-10-4)

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