GWR pannier tank

The GWR 0-6-0PT (Pannier Tank), is a type of steam locomotive built by the British Great Western Railway with the water tanks carried on both sides of the boiler, in the manner of panniers. They were used for local, suburban and branch line passenger and goods traffic, for shunting duties, and as banker engines on inclines. The early examples, such as the 1901 and 2021 classes, were rebuilt from saddle or side tanks when the locos received a Belpaire firebox – this type of firebox has a square top and is incompatible with a curved saddle tank. This process mostly took place during the tenure at Swindon Works of George Jackson Churchward. Only a very small number of saddle tank locomotives escaped rebuilding as panniers, notably the 1361 Class built new under Churchward in 1910, by which date a few of the 1813 Class had already been rebuilt as pannier tanks.

The GWR pannier tank locomotives were classified as follows:

Small engines (wheelbase under 15'), rebuilt from saddle or side tanks

Small engines (wheelbase under 15'), built as pannier tanks from new

Large engines (wheelbase over 15'), rebuilt from saddle or side tanks

Large engines (wheelbase over 15'), built as pannier tanks from new

Large boiler/short wheelbase

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Wikipedia