Derby Lightweight Railcar / "Iris"

In the early 1950s British Railways built a number of DMUs which became known as "Lightweights" due to the method of construction from light alloy sheets. The first batch used Leyland engines and torque converter transmission but these were soon joined by a second batch which used BUT (AEC) engines and a pre-selector gearbox. This later batch proved to be much more what BR wanted and they set the standard for DMUs [1] for the next 40 years and beyond (indeed Class 121s are still in revenue earning service!)

As well as a number of 2- and 4-car sets a couple of single railcars were also built for the Banbury (Merton Street) to Buckingham/Bletchley route. However being non-standard they were withdrawn (like the other Lightweights) after just a few years in service [2] by the end of the 1960s.

Information
Number built: 2
Built: 1956
Builder: British Railways Derby Works
Engine: 2 BUT (AEC) horizontal 6-cylinder diesels
Power: 300 hp (224 kW)
Formation: Driving Motor Brake Standard (DMBS)

However some cars avoided scrapping and went into departmental service, one of the single cars became QXV RDB975010 Iris when it was transferred in 1967 to British Rail Research [3]. Following rebuilding it was used for radio control and survey research at Derby [4] later it was used for radio propagation, noise measurement and data transmission tests [5]. Now it has been preserved at the Ecclesbourne Valley Railway and has been restored to it's original condition.
Iris at Duffield

Iris at Wirksworth

[1] Brian Haresnape, Diesel Multiple Units: The First Generation (Ian Allan, 1985) p. 23
[2] Colin J Marsden, DMU and EMU Recogition Guide (Ian Allan, 2013) p. 28
[3] Colin J Marsden, Twenty Five Years of Railway Research (OPC, 1989) p. 96 
[4] Haresnape p. 25
[5] Colin J Marsden, Departmental Stock (Ian Allan, 1984) p. 34