CSAR Rack 4-6-4RT

From Wikipedia, the free encyclopedia
Jump to navigation Jump to search

CSAR Rack 4-6-4RT
CSAR Rack 4-6-4T no. 996.jpg
CSAR rack locomotive no. 996, c. 1906
Type and origin
Power typeSteam
DesignerCentral South African Railways
(P.A. Hyde)
BuilderVulcan Foundry
Serial number1942-1943
ModelCSAR Rack
Build date1905
Total produced2
RebuilderCentral South African Railways
Rebuild date1906
Number rebuilt2
 • Whyte4-6-4RT (Baltic)
 • UIC2'C2'n4t as built
2'C2'n2t modified
Driver2nd coupled axle
Gauge3 ft 6 in (1,067 mm) Cape gauge
Leading dia.30 in (762 mm)
Coupled dia.42 14 in (1,073 mm)
Trailing dia.30 in (762 mm)
Wheelbase33 ft 7 in (10,236 mm)
 • Leading6 ft 4 in (1,930 mm)
 • Coupled12 ft 9 in (3,886 mm)
 • Trailing6 ft 4 in (1,930 mm)
Wheel spacing
1-2: 8 ft 3 in (2,515 mm)
2-3: 4 ft 6 in (1,372 mm)
 • Over couplers41 ft 5 in (12,624 mm)
Height13 ft (3,962 mm)
Frame typePlate
Axle load15 LT 15 cwt (16,000 kg)
 • Leading18 LT (18,290 kg)
 • Coupled15 LT 15 cwt (16,000 kg)
 • Trailing19 LT (19,300 kg)
Adhesive weight47 LT 5 cwt (48,010 kg)
Loco weight84 LT 5 cwt (85,600 kg)
Fuel typeCoal
Fuel capacity96 cu ft (2.7 m3)
Water cap1,200 imp gal (5,500 l)
Firebox typeRound-top
 • Firegrate area33.5 sq ft (3.11 m2)
 • Pitch7 ft 6 in (2,286 mm)
 • Diameter5 ft (1,524 mm)
 • Tube plates12 ft 7 in (3,835 mm)
 • Small tubes197: 2 in (51 mm)
Boiler pressure200 psi (1,379 kPa)
Safety valveRamsbottom
Heating surface1,438.33 sq ft (133.625 m2)
 • Tubes1,298.13 sq ft (120.600 m2)
 • Firebox140.2 sq ft (13.03 m2)
CylindersFour as built, two modified
Cylinder size18 in (457 mm) bore
20 in (508 mm) stroke
Valve gearJoy
Valve typeBalanced slide
Loco brakeCounter-pressure steam
Train brakesVacuum
CouplersJohnston link-and-pin
Performance figures
Tractive effort23,000 lbf (100 kN) @ 75%
OperatorsCentral South African Railways
South African Railways
ClassCSAR Rack
Number in class2
NumbersCSAR 995-996
SAR O995-O996
First run1905

The Central South African Railways Rack 4-6-4RT of 1905 was a South African steam locomotive from the pre-Union era in Transvaal Colony.

In 1905, the Central South African Railways placed two four-cylinder rack tank locomotives with a 4-6-4 Baltic type wheel arrangement in service on the section between Waterval Onder and Waterval Boven. The locomotives were underpowered and proved to be failures in rack service, with the result that their rack mechanisms were removed in 1906 to convert them to ordinary two-cylinder tank locomotives.[1][2]


In 1905, the Central South African Railways (CSAR) placed orders with Vulcan Foundry for the construction of two 4-6-4 Baltic type rack tank locomotives for use on the rack section between Waterval Onder and Waterval Boven. According to the designs and specifications of CSAR Chief Locomotive Superintendent P.A. Hyde, the CSAR Rack locomotive was to have a bar frame, two cylinders with 18 inches (457 millimetres) bore and 30 inches (762 millimetres) stroke, and was to use the same drive system as the Nederlandsche-Zuid-Afrikaansche Spoorweg-Maatschappij 32 Tonner, CSAR Class G, which had proven satisfactory in rack service. The two locomotives were delivered in 1905, numbered 995 and 996, following on from the number range of the Class G rack locomotives.[1]

The builders chose not to adhere to the original designs and specifications. The cylinder stroke was reduced to 20 inches (508 millimetres) and two inner cylinders of the same dimensions were added to drive the rack mechanism independently. A plate frame was used instead of the specified bar frame. The main plate frames were 1 14 inches (32 millimetres) thick and were arranged outside the coupled wheels. The balanced flat slide valves were actuated by Joy valve gear.[1][2][3]

The boiler capacity was not increased to compensate for the additional cylinders and, in service, trouble was experienced under working conditions. Tests showed that the locomotives could not maintain steam and had insufficient adhesive weight to prevent slipping on the steep gradient. The boiler had to generate the supply of steam while the engine passed through the tunnel at the head of the incline with the blast pipe not functioning, since the exhaust steam was diverted to the side tanks while the counter-pressure air brake system was in operation.[1][2][3]


Even though it turned out to be a failure in service, the locomotive design had several novel features at the time.[1]

Rack mechanism[edit]

The outside cylinders drove the coupled wheels and the inside cylinders drove a coupled pair of rack wheels. The rack wheels were carried on a frame suspended from the widely separated leading and driving coupled wheel axles, which had an 8 feet 3 inches (2,515 millimetres) wheelbase.[1][2]

Rather than the bar frame as specified, an outside plate frame was used to accommodate the inside cylinders. The connecting rods of the inside engine were not connected directly to the crank pins, but to projections on the coupling rods which actuated the rack gear. This method of coupling was unavoidable due to the restricted width of only 3 feet 3 inches (991 millimetres) between tyres.[1][2]

To compensate for tyre wear on the coupled driving wheels, the rack axle bearings could be adjusted vertically, while the rack wheel teeth were of involute form to ensure correct action between adjustments. The inner cranks were of the disk type, with triangular circumferential grooves to take the cast-iron brake blocks of the rack engine hand brake.[1][2]

Brake systems[edit]

The valve gear of the outside and inside sets of motion could each be separately reversed by its own screw gear, while each set also had its own independent regulators, injectors and feed pumps. The locomotive had steam brakes on all coupled wheels as well as both bogies, the first and last South African steam locomotive to be equipped with braked bogies. The coupled wheels and the rack engine's crank disks had separate hand brakes and the inside and outside piston pairs had separate counter-pressure air brakes. In addition, the locomotive had a combination ejector to work the train's vacuum brakes.[1][2]

The Le Chatelier counter-pressure air brake system made use of the following:

  • A valve in the base of the blastpipe isolated the cylinders from the smokebox to prevent the ingress of hot gases and cinders when the motion was reversed.
  • Air was drawn from outside the smokebox through a pair of non-return valves into the exhaust ports, from where it was compressed into the steam chests.
  • The compressed air was allowed to escape into the atmosphere through a graduating discharge valve and a silencer, mounted behind the chimney.
  • To take up the heat of compression and prevent overheating of the cylinders, a small water jet delivered a cold spray into the exhaust passages.[1][2]


To protect the crew from the smoke and gases in the tunnel, the large cab was totally enclosed and had side doors and windows.[1][2]


Central South African Railways[edit]

Converted rack no. 0995, circa 1912

In spite of its advanced features, the CSAR Rack locomotive was a failure and was outperformed by the older and much smaller predecessor Class G rack locomotive. As a result, the rack mechanisms of both locomotives were removed within a year of their entering service, converting them to ordinary two-cylinder tank locomotives. Owing to their failure, the old 32 Tonners remained in service until 1908, when a new alignment with easier gradients, including a new tunnel, was constructed and the rack section could be abandoned.[1][2][3][4]

Their failure can be attributed directly to the fact that two additional cylinders were introduced by the builders while the boiler capacity remained the same, which resulted in a locomotive with an inadequate steaming ability. In addition, the complicated rack mechanism led to perpetual maintenance problems.[1][2][3]

South African Railways[edit]

When the Union of South Africa was established on 31 May 1910, the three Colonial government railways (Cape Government Railways, Natal Government Railways and CSAR) were united under a single administration to control and administer the railways, ports and harbours of the Union. Although the South African Railways and Harbours came into existence in 1910, the actual classification and renumbering of all the rolling stock of the three constituent railways were only implemented with effect from 1 January 1912.[5][6]

In 1912, the locomotives were considered obsolete and were not classified or renumbered, but instead only had the numeral "0" prefixed to their existing numbers. Both locomotives were withdrawn from service by 1915, a mere ten years after being built.[6][7]


  1. ^ a b c d e f g h i j k l m Holland, D.F. (1971). Steam Locomotives of the South African Railways. 1: 1859–1910 (1st ed.). Newton Abbott, Devon: David & Charles. pp. 135–137. ISBN 978-0-7153-5382-0.
  2. ^ a b c d e f g h i j k Espitalier, T.J.; Day, W.A.J. (1945). The Locomotive in South Africa - A Brief History of Railway Development. Chapter VI - Imperial Military Railways and C.S.A.R. (Continued). South African Railways and Harbours Magazine, February 1945. pp. 99-100.
  3. ^ a b c d Soul of A Railway, System 8, Part 1: Pretoria: including local services, workshops and running sheds, Part 1. Introduction. (Accessed on 15 March 2017)
  4. ^ Espitalier, T.J.; Day, W.A.J. (1944). The Locomotive in South Africa - A Brief History of Railway Development. Chapter IV - The N.Z.A.S.M. (Continued). South African Railways and Harbours Magazine, November 1944. pp. 843-844.
  5. ^ The South African Railways - Historical Survey. Editor George Hart, Publisher Bill Hart, Sponsored by Dorbyl Ltd., Published c. 1978, p. 25.
  6. ^ a b Classification of S.A.R. Engines with Renumbering Lists, issued by the Chief Mechanical Engineer's Office, Pretoria, January 1912, pp. 11, 13, 18 (Reprinted in April 1987 by SATS Museum, R.3125-6/9/11-1000)
  7. ^ Paxton, Leith; Bourne, David (1985). Locomotives of the South African Railways (1st ed.). Cape Town: Struik. p. 23. ISBN 0869772112.