Wikipedia
Dynamic braking (locomotive)
Dynamic braking is the use of the electric traction motors of a railroad vehicle as generators when slowing the locomotive. It is termed rheostatic if the generated electrical power is dissipated as heat in brake grid resistors, and regenerative if the power is returned to the supply line. Dynamic braking lowers the wear of friction-based braking components, and additionally regeneration can also lower energy consumption. Dynamic braking can also be used on railcars with multiple units, light rail vehicles, trams and PCC streetcars.