The 9400 class was the final development in a long lineage of Tank Locomotives that can be directly traced to the 645 Class of 1872. Over the decades details altered, the most significant being the adoption of Belpaire Fireboxes necessitating pannier tanks.
The 9400 resembled a pannier tank version of the 2251 class, and indeed shared the same boiler and cylinders as the 2251, but was in fact a taper- boilered development of the 8750 subgroup of the 5700 class. The advantage was a useful increase in boiler power, but there was a significant weight penalty that restricted route availability.
The 10 GWR-built locomotives had superheaters but the remainder did not. The first ten 9400s were built by the Great Western and were the last steam engines built by the company. After the Nationalisation to BR in 1948, private contractors built another 200 for BR.
The 9400s were numbered 9400–9499, 8400–8499 and 3400–3409. BR gave them the power classification 4F.