Great Western Railway Weight Classification

From 1920, the cab side of Great Western Railway (GWR) steam locomotives bore a letter on a coloured disc, which enabled staff to quickly assess the capabilities of locomotives without the need to check tables of data. The letter showed the power classification, and the coloured disc showed the weight restriction. This system continued after the GWR became the Western Region of British Railways.

On 1 July 1905 the Great Western Railway (GWR) introduced a system for denoting both the haulage capabilities and the weight restrictions which applied to their various classes of locomotive. Originally this was used only in record books, but from mid 1919 began to be shown on the locomotives themselves. The weight restriction was shown in the form of a coloured disc, and the power classification was denoted by a capital letter placed upon the disc. At first these were painted high on the cab side, but during World War II the blackout precautions meant that staff had to be careful using lights at night, so the disc and letter were moved downwards to a position just above the engine number plate, to make them easier to see.

When locomotives were loaned by other railways to the GWR during World War II, these were also allotted GWR power and weight classifications, so that GWR staff responsible for locomotive rostering could select the most suitable engine for the task without needing to learn an unfamiliar system.

The GWR was nationalised in 1948, becoming the Western Region of British Railways. In 1949, BR decided to adopt the London, Midland and Scottish Railway (LMS) system of power classification for all locomotives. Despite this, the use of a letter to denote power classification continued to be used on former GWR steam locomotives, as did the coloured disc for weight classification; both continued until the end of steam traction on the Western Region in 1965. Certain ex-LMS and BR Standard steam locomotives allocated to the Western Region were given GWR style route classification discs, usually without the power class letter. Some of these had the BR power class shown on the disc as a figure; for example, the Class 5 4-6-0 bore the figure "5" on a red disc. The coloured disc was also applied to some diesel locomotives allocated to the Western Region.

...
Wikipedia