Over the decades details altered, the most significant being the adoption of Belpaire fireboxes necessitating pannier tanks.

The advantage was a useful increase in boiler power, but there was a significant weight penalty that restricted route availability.

[3] The 9400 Class migrated to most parts of the former GWR, with many based in South Wales and at Old Oak Common.

A common sight on the departure side in 1964–1965 was a worn down 9400 Class locomotive without number plates waiting with a line of Mark 1 coaches.

[citation needed] Numbers 8400 to 8406 served as bank engines on the Lickey Incline after its transferral to the Western Region.