The electrified suburban lines were busy with commuter traffic which took priority over freight workings, and the London network abounded with junctions and signal stops.

The design brief called for a powerful locomotive with good acceleration and adhesion:[2] one that was able to climb the steep gradients over the flying junctions that were to be found all over the Southern Railway's London network.

The ability to operate over tight curves and be able to pull away from a standstill on the aforementioned gradients were also key considerations that needed to be incorporated into the design.

The SR Z class was considered at first, but due to the specialist characteristics of the design as a yard engine for marshalling freight, they were not deemed fit for the purpose of hauling loads under the tight timings of London's railway system.

The solution was a smaller wheeled version of the ill-fated K1 River class tank engines, with three cylinders to allow for better acceleration and three sets of Walschaerts valve gear.

They were also used on inter-company/regional transfer freight duties over the West London Line to Old Oak Common, Willesden, Cricklewood and Ferme Park marshalling yards.

[7] The class was generally well liked by their crews apart from the fact that the driver was on the right hand side of the Eastleigh batch, which made accurate stopping difficult when signalling was predominantly on the left on the Southern network.

In 1948 there was a chronic shortage of large passenger tank locomotives on the un-electrified lines of the former London Brighton and South Coast Railway.