Watson disliked articulated locomotives and his aim with the Class 20 was to build an as powerful as possible non-articulated locomotive with a maximum axle load.
The construction of this sole Class 20 locomotive marked the beginning of a research process which was continued two years later with the construction of the sole Class 21 2-10-4 and which eventually culminated in the procurement of the Class 23 4-8-2 fleet in 1939 [REF].