Dean Bogie Dynamometer Car (SPEC) Royal Clerestory (Diagram M18 and Q20)

Number range: 790 (1). Built Swindon 1901 to Wagon Lot 293. One Machine and One Working Compartment with One Toilet.

Designed for measuring real world engine performance and was marshalled behind the locomotive while the technicians in the coach monitored speed, steam pressure, coal and water consumption, tractive effort.

Renumbered W7W with the Pilot Vans in 1921. Withdrawn at Swindon in 1961. Preserved in 1965. 

The retractable, flangeless wheel for recording speed and all other recording equipment no longer survives.