When the London and South Western Railway urgently needed locomotives for heavy freight trains, chief engineer Robert Urie designed the H15 class as a 4-6-0 locomotive with six feet diameter coupled wheels. The same axle configuration and wheel diameter had been used on the F13 in 1905, but the H13 only had two large cylinders on the outside. By also arranging the valve gear on the outside, easy access to all important parts was achieved, which is advantageous for freight locomotives. While Urie was a proponent of superheated steam technology, he was unsure of the best superheater design. For this reason he decided to build different versions of the locomotive.
The first series consisted of four units with Schmidt-type superheaters, four with Robinson-type superheaters and two without any superheaters for comparison. As neither design fully met Urie's expectations, he developed his own superheater and named it the “Eastleigh Superheater” after the factory that built the locomotives. The first locomotive to receive this was converted from the unsuccessful E14 class one-off. Ultimately, the first ten pieces were retrofitted with these superheaters. After the grouping, 15 more examples were built on the Southern Railway under Richard Maunsell. Five of these were created as conversions from the F13 class locomotives.
Schematic drawing with dimensions
Locomotive Magazine, December 1913
In addition to easy maintenance, good performance and very little wear were also evident in operation. Only the examples converted from the F13 had a very high cab, which presented particular challenges for personnel under six feet tall and earned these locomotives the nickname “Cathedrals”. The originally green and ornate color scheme was changed to a plain black from 1939 under Bulleid. When British Railways was founded, they were classified in the 4P5F power class, i.e. power class four in passenger service and power class five in freight service. The decommissioning of the H15 was completed in 1961, so not a single one was preserved.