velocity. Power would later be manually calculated from these data on early cars. Some later cars were equipped with a mechanical integrator to directly record the power. A separate use for the car was to test a particular rail route to rate it for tonnage based on a run with a dynamometer car and recording the effect of the grades and curvature on the capacity and resulting power requirements for that line. Power calculations The operating principle of the dynamometer car is based on the basic equation for power being equal to force times distance over time. This equation can be reduced to power equals force times velocity: In other words, the instantaneous power output of the locomotive can be calculated by measuring the pull on the coupler and multiplying by the current speed. Converting to horse power gives: References External links Testing a Locomotive — Comprehensive details of how a dynamometer car is used for performance testing Rail Training & Test Cars photos of some prototype cars. Carriages of LNWR: 6-wheeled Dynamometer Car Laboratory on Wheels Details (including publicity article from February 1951) of the Victorian & South Australian Railways' dynamometer car Esoteric Test Equipment Norfolk & Western dynamometer car drawings Maintenance of way equipment Dynamometers Charles Babbage