After the First World War, George Churchward had already designed an express locomotive that was intended to complement and surpass his Star class. It was to receive the GWR standard boiler number 7, which was already used on the class 4700 2-8-0 express goods locomotive. However, this plan could not be implemented as it would have exceeded the permitted axle load of 20 long tons. 

Only his successor, Charles Benjamin Collett, designed the Castle class in the 1920s. The standard boiler number 8 was developed for this, which was lighter with increased size and had a larger grate area. It was tapered towards the front and had no dome. The associated locomotive was based on the Star class and had an extended frame with the same wheel diameter. This meant that the longer Belpaire firebox could be better accommodated and a larger driver's cab with side windows could now be used. In view of the larger boiler output, the diameter of the four cylinders was increased by one inch.

Schematic drawing with dimensions

Locomotive Magazine, September 1923

They replaced the Star class in use in front of the heavier express trains and were able to maintain high average speeds thanks to their 80.5-inch wheels and high boiler output. Shortly after they were put into service, they had competition from the even more powerful King class, which, however, was not as fast due to the smaller wheels. Thus, the heaviest express trains were pulled by the King class, while the Castle class got the faster trains that weren't quite as heavy.

The Castle class was considered Britain's most powerful express locomotive at the time of its introduction, although Gresley's A3 at the LMS was larger and also claimed that title. In comparison runs in 1924, however, the Castle class was able to win in the areas of tractive effort, average speed and fuel consumption. The Cheltenham Flyer, which weighed only 180 long tons, was able to cover the 77.25 mile-long route in 1932 at an average of 81.68 mph. The top speed reached 89 mph. 

Locomotive Magazine, September 1923

Over a long period from 1923 to 1950, 155 Castle class locomotives were built. A further 16 examples were created from the conversion of other locomotives. In addition to some examples of the Star class, this also included the Pacific “The Great Bear”, which was unsuccessful and was therefore converted back into a 4-6-0. Most machines were named after castles, but with a few exceptions.

The number 5005 “Manorbier Castle” was streamlined in 1935 and reached 100 mph. However, since this hardly brought any advantages in everyday life, the paneling was soon removed again. Later modifications to the production machines included a higher degree of superheating and, in some cases, a double chimney. In this way, No. 7018 “Drysllwyn Castle” reached a speed of 102 mph in 1958. Eight pieces are still preserved today, six of which had been operational at one point in time. In 2019, only one of these was still operational, while four others were currently being refurbished.