Power transmission on diesel traction

Since, unlike steam engines, diesel engines cannot develop the maximum torque needed to move railway vehicles from a standing start, the required tractive power has to be provided via a special transmission system. Power can be transmitted from the diesel to the driving wheelsets either mechanically via gears, hydraulically via a fluid or electrically using DC or three-phase current. Purely mechanical drives are no longer used nowadays, having been superseded by hydromechanical systems. Hydraulic drive can be effected using a hydrostatic or hydrodynamic set-up, the two complementing each other in traction and auxiliary equipment on diesel locomotive units. Hydrodynamic drive is commonest on railcars, while electric transmission is most often used for mainline locomotives, either with conventional DC technology or, increasingly, the modern alternative of power electronics or three-phase traction technology.