This class of locomotive was first placed in service in June 1936 and 184 units were built to deal with heavy mixed traffic working. As originally designed they had a long record of successful performance. In order to assist in their more intensive utilisation,. self-cleaning plates were introduced into the smokebox from 1946, which resulted in a deterioration in steaming capacity far below the previous standard, which could not be recovered in spite of a number of trial variations in the blastpipe and chimney proportions, including reduction of the blastpipe orifice from 5.1" to 5".

Since similar trouble had been experienced elsewhere in applying self-cleaning plates to the smoke boxes of 3-cylinder as distinct from 2-cylinder engines, it was decided to send a representative engine to the Swindon Test Plant for steaming tests as then running and with such modification as would result in improved performance. 

As a result of relatively small modifications to blastpipe and chimney dimensions the rate of continuous evaporation which could be sustained was more than doubled and in its final condition opportunity was taken to carry out a series of full scale performance and efficiency test which were the subject of this Bulletin. 

The presentation of the data in the report was divided into two main parts: 

The first defined the relationship between coal as fired, water as drawn from the tender, tractive effort and horsepower both as available at the drawbar, data directly applicable to the immediate commercial purpose of examining train loadings and schedules to obtain reduction in fuel consumption by working the locomotives where possible nearest their point of maximum operating efficiency. 

The second part concerned itself mainly with thermal efficiency, giving data on a basis of indicated power, covering boiler and cylinder efficiencies, factors of more importance in locomotive design.

The tests were carried out under the direction of Mr. R.A.Riddles, Member (Mechanical & Electrical Engineering) Former Railway Executive, the work being controlled by the Locomotive Testing Committee. 

The static testing took place on the test rig at Swindon and the road tests between Bristol SPM and Reading via the Badminton line.

Here are a few pictures of the tests and a summary graph.