I photographed this 1906 Dynamometer Car North Eastern Railway #902502 at the National Railway Museum in York England.
Dynamometer Cars were used to scientifically measure the performance of trains in order to effect improvements. On 3 July 1938, a Dynamometer Car recorded the world speed record for steam locomotives at 125.88 mph, set by the 1938 4-6-2 A4 Class #4468 “Mallard”