The first class of tank locomotives that the GWR had built from the beginning as “Pannier Tanks” was the class 5700. Shortly after this came the lighter class 5400. The special shape of the water tanks got its name from the panniers which were used by pack animals. These were easier to adapt to the Belpaire firebox compared to a saddle tank and resulted in a lower center of gravity.

The class 5700 came from Collett and was based on the class 2721 locomotives, which were later rebuilt into Pannier Tanks. A total of 863 were built between 1929 and 1950, making them the most numerous class of the GWR and thus became an icon. Some adapted variants were designated the classes 6700, 8750 and 9700. Due to the high number of locomotives, other number ranges also had to be used, such as the number 3738 shown in the photo.

Although heavier than the class 5400, the class 5700 could also be used on 70 percent of the GWR route network. Since the hammer blow was comparatively light, they were later able to operate on 90 percent of the network due to a reassessment of the axle load class. It was used not only as a freight and shunting locomotive, but also regularly as a passenger locomotive. Their retirement took place between 1956 and 1966.