South African Class 6Z 2-6-4

CGR 6th Class 2-6-4 South African Class 6Z 2-6-4
CGR 6th Class 2-6-2 Prairie as built, before modification to 2-6-4 Adriatic
Type and origin Power type Steam Designer Cape Government Railways (H.M. Beatty) Builder Neilson, Reid and Company Serial number 5867-5870, 6075-6078 Model CGR 6th Class 2-6-4 Build date 1901 Total produced 8 Rebuilder Cape Government Railways Rebuild date 1902 Number rebuilt 4 from 2-6-2 (Prairie)
Specifications Configuration: ​  • Whyte 2-6-4 (Adriatic)  • UIC 1'C2'n2t Driver 3rd coupled axle Gauge 3 ft 6 in (1,067 mm) Cape gauge Leading dia. 28 1⁄2 in (724 mm) Coupled dia. 54 in (1,372 mm) Trailing dia. 28 1⁄2 in (724 mm) Tender wheels 37 in (940 mm) Wheelbase 45 ft 9 1⁄8 in (13,948 mm)  • Engine 28 ft (8,534 mm)  • Coupled 9 ft 10 in (2,997 mm)  • Trailing 5 ft (1,524 mm)  • Tender 10 ft (3,048 mm) Wheel spacing (Asymmetrical) 1-2: 5 ft 1 in (1,549 mm) 2-3: 4 ft 9 in (1,448 mm) Length: ​  • Over couplers 55 ft 7 1⁄4 in (16,948 mm) Height 12 ft 10 in (3,912 mm) Frame type Bar Axle load 12 LT 17 cwt (13,060 kg)  • Leading 5 LT 19 cwt (6,045 kg)  • 1st coupled 12 LT 17 cwt (13,060 kg)  • 2nd coupled 11 LT 3 cwt (11,330 kg)  • 3rd coupled 12 LT 1 cwt (12,240 kg)  • Trailing 12 LT 8 cwt (12,600 kg)  • Tender axle Axle 1: 10 LT 8 cwt (10,570 kg) Axle 2: 10 LT 15 cwt (10,920 kg) Axle 3: 10 LT 17 cwt (11,020 kg) Adhesive weight 36 LT 1 cwt (36,630 kg) Loco weight 54 LT 8 cwt (55,270 kg) Tender weight 32 LT (32,510 kg) Total weight 86 LT 8 cwt (87,790 kg) Tender type YE (3-axle) Fuel type Coal Fuel capacity 5 LT 10 cwt (5.6 t) Water cap 2,825 imp gal (12,800 l) Firebox type Round-top  • Firegrate area 25.5 sq ft (2.37 m2) Boiler: ​  • Pitch 6 ft 10 in (2,083 mm)  • Diameter 4 ft 7 in (1,397 mm)  • Tube plates 13 ft 10 7⁄8 in (4,239 mm)  • Small tubes 188: 2 in (51 mm) Boiler pressure 170 psi (1,172 kPa) Safety valve Ramsbottom Heating surface 1,462 sq ft (135.8 m2)  • Tubes 1,366 sq ft (126.9 m2)  • Firebox 96 sq ft (8.9 m2) Cylinders Two Cylinder size 18 in (457 mm) bore 26 in (660 mm) stroke Valve gear Stephenson Couplers Johnston link-and-pin
Performance figures Tractive effort 19,890 lbf (88.5 kN) @ 75%
Career Operators Cape Government Railways South African Railways Class CGR 6th Class 2-6-4 SAR Class 6Z Number in class 8 Numbers CGR 270-277, SAR 713-720 Delivered 1901-1902 First run 1901 Withdrawn 1934

The South African Railways Class 6Z 2-6-4 of 1901 was a steam locomotive from the pre-Union era in the Cape of Good Hope.

In 1901, the Cape Government Railways placed four 6th Class steam locomotives with a 2-6-2 Prairie type wheel arrangement in service. The engines were soon modified to a 2-6-4 Adriatic type wheel arrangement. In 1902, another four locomotives were placed in service, built with the 2-6-4 wheel arrangement. These latter four were the first tender locomotives in the world to be built with this wheel arrangement. In 1912, when they were assimilated into the South African Railways, these eight locomotives were renumbered and designated Class 6Z.

The Cape 6th Class 2-6-4 locomotive was designed at the Salt River works of the Cape Government Railways (CGR). It was a further development of the very successful 6th Class 4-6-0 locomotive, based on the experience gained with the several British and American versions of the Class and incorporating features from both. The design began as an experimental redesign of the 6th Class with a bar frame and wide firebox and with a 2-6-2 Prairie type wheel arrangement.

Good though the 6th Class 4-6-0 locomotives were, the requirement grew for larger and more powerful engines. At the time, it was becoming increasingly apparent that such locomotives would require fireboxes with appreciably larger grate areas. This was not possible with the existing type of frame on a Cape gauge locomotive where the firebox was arranged between the rearmost coupled wheels. Longer fireboxes were impractical since it would be accompanied by greater fatigue to firemen as well as difficulties in design. To arrange the firebox over the coupled wheels would be unsuitable for adoption on larger locomotives, given the limitation on the maximum height of the boiler centreline above the railhead which was in vogue at the time.

To overcome this problem, CGR Chief Loco­mo­tive Superintendent H.M. Beatty prepared designs for a new locomotive with a 2-6-2 wheel arrangement of which the frame terminated in front of the firebox, where it connected to a casting termed the "bridle casting". This casting extended out on either side and had wider spaced frames attached to its rear, which allowed a wider and deeper firebox with an interior width of 49 ¹⁄₄ inches (1,251 millimetres).