Dynamometer car

North Eastern Railway Dynamometer Car at the National Railway Museum in York
Victorian and South Australian Railways joint stock Dynamometer Car (coupled between the locomotive tender and the train) being used to record the performance of a VR X class locomotive running on pulverised brown coal.

A dynamometer car is a railroad maintenance of way car used for measuring various aspects of a locomotive's performance. Measurements include tractive effort (pulling force), power, top speed, etc.