The side mounted pannier tank design engines were a distinctive feature of the GWR tank locomotive design in the 20th century. The pannier tanks were an adaptation of the saddle tank engines the GWR had built in the 19th century. Eliminating the tank over the top of the boiler allowed boilers with the more efficient belpaire firebox to be fitted while retaining access under the tank to the cylinders, rods and valve gear between the frames. Squaring off the tanks helped mitigate the loss of water capacity. In addition to the new locomotives being built many of the existing saddle tank engines were altered to pannier tank format to accommodate one of the new standard belpaire firebox boilers.
By the late 1920s the batches of 0-6-0 tank engines which had been built in the 1870s-90s were reaching the end of their economic service lives. Their replacements were almost identical, excepting the design was thoroughly updated to produce a new standard class, numbered from 5700, which was to reach 863 locomotives. Naturally there were many detail variations but only 2 significant variants. The first 300 locomotives were built with a traditional cab roof profile and backsheet, a style inherited from the 19th century earlier open cab arrangements.
Next came a small batch of locomotives with the pannier tanks extended down to the footplate and fitted with condensing pipes specifically for service underground on the Metropolitan Railway lines in London numbered 9701 to 9710. These engines featured the new rounded Collett cab design, giving the engines a much more modern appearance. Alongside 9701 the next batch of the standard 57xx class pannier tanks was being started and the revised cab design was used on all (except one!) of the engines from 8750 (September 1933), amounting to another 552 locomotives.
The odd one out was locomotive 8700, which conspired to be built twice. The first 8700 (Beyer Peacock 1931) was used for trails with the condensing equipment and when the 9700 sub-class was built in late 1933 an additional set of tanks and cab was constructed. 8700 was renumbered 9700 in January 1934. The last locomotive from the 8750 batch completed in March 1934 should have become 8799 but was instead fitted with the original pannier tanks and old-style cab from 8700, becoming 8700(2) and later being one of the engines known to have returned to Swindon after withdraw still carrying GWR livery with the 1934 shirtbutton logo.
In the mid-1950s London Transport found many of it's former Metropolitan Railway steam locomotives were almost worn out, but were still needed to haul engineering trains. Diesel traction was proving problematic, shunters like the BR class 08 were too slow while mainline diesel types exceeded the Metropolitans' loading gauge. After trails the GWR 57xx panniers proved ideal, fast enough to keep out of the way of the electric trains and a redundant BR engine was a lot cheaper than the other options. 13 ex-GWR 57xx panniers were to serve London Transport (though not all at the same time) and these 'red panniers' were to become the last British steam locomotives in mainline service in 1971. Six of the ex-LT locomotives entered preservation including the Railway Children locomotive 5775.