Control systems for d.c. traction motors are well established. The developments in power converters, particularly GTO thyristors and IGBTs, and the use of microprocessor controllers have produced control systems which offer a wide and flexible control range with efficient power conversion, whilst utilising relatively simple control techniques. The main limitation is the motor commutator which requires maintenance and restricts high speed performance. However, the continuing developments in a.c. induction motor control systems are resulting in the railway industry increasing its use of this technology. Although this trend will continue, there are a large number of vehicles fitted with d.c. motors and electromechanical controllers. In order to increase the life of such equipment, it has been found economical to upgrade the control systems using techniques which have been described here. Such systems are likely to be in use for many years in the future.