| Weight | 180t 18cwt |
| Driving Wheels | 4ft 3ins |
| Boiler Pressure | 200psi Superheated |
| Cylinders | Four Outside – 20in x 26in |
| Tractive Effort | 42,740lbf (15A 47,490lbf) |
The 20th class was the only class of Rhodesia Railways locomotives to be fitted with mechanical stokers.
All of the 47 locomotives were built by Beyer, Peacock and Company at Gorton, Mancherster.
- 15 in 1954 which were numbered 700-714
- 6 in 1957 which were numbered 715-720
- 30 in 1957 which were numbered 715-744
- 16 in 1957/58 which were numbered 745-760
The later locomotives (721-760) were classified as class 20A; the main difference being that the trailing wheels were larger and the locomotives were heavier.
The initial use of the locomotives was ill-fated as they were plagued with teething troubles including cracks in the locomotives’ frames and fireboxes. Unlike the South African Railways’ GMA, which had cast steel frames, theses had conventional fabricated bar frames.
Two locomotives were scrapped very early due to accident damage. Strangely, they were the first (700) and the last (760) built.
Apart from these problems, the locomotives met all expectations. They were by far the most powerful locomotives on the Rhodesia Railways capable of hauling 1,250 ton load up a 1 in 64 incline.
In 1964, Zambia took over the operation of railways in its territory. This included the transfer of 80–90 locomotives – about half of them 20th and 20A class locomotives.
By June 1975, there were only 18 in service in Rhodesia, all allocated to Bulawayo.
In Zambia, the locomotives were replaced by diesels and retired by about 1970.
In 1979 / 1980 Zambia briefly placed four back into service due to an oil shortage.
In 1978 Rhodesia Railways started a rebuilding program for its steam locomotive fleet. Between 1980 and 1983 the remaining Garratt locomotives were completely overhauled and had some modernisation, including the installation of roller bearings. The work was undertaken by private companies, especially the RESSCO works in Bulawayo.
Work on the first two was completed in 1980, although it was April 1983 before they were all dealt with.
As part of the rebuilding programme, the locomotives’ running numbers were consolidated into two blocks: class 20 (730–737) and class 20A (740–750). Three locomotives kept their old numbers: 746, 747, and 749.
All of the rebuilt locomotives were allocated to Bulawayo.
Due to the continuing difficult economic situation in Zimbabwe, the rebuilt steam locomotives remained in service longer than originally planned.
Finally, in the early 1990s, new diesels were ordered and the locomotives were systematically withdrawn as they became due for general overhaul. All were withdrawn from general service by 1994.
All those allocated to Bulawayo with the exception of 730/740/742/749 (retained for museum activity and spare parts) were then put up for tender and scrapped as part of the contract to purchase Canadian diesel locomotives. 736 was later saved,
Preservation
Beyer Peacock Works No 7690 – RR 705 & 730 Isuga
Beyer Peacock Works No 7693 – RR 708
Beyer Peacock Works No 7801 – RR 736
Beyer Peacock Works No 7805 – RR 740
Beyer Peacock Works No 7806 – RR 741
Beyer Peacock Works No 7807 – RR 742
Beyer Peacock Works No 7814 – RR 749
Beyer Peacock Works No 7823 – RR 758
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Preserved British Steam Locomotives 