A project is being carried out by British Rail R & D Division to examine the feasibility of using active control to improve the performance of secondary suspensions for railway vehicles. The suspensions are based upon adding a system of actuators to a vehicle having a conventional passive suspension, and applying control by means of these actuators in order to modify the suspension's performance in response to the inputs from the track. The Controller causes the suspension's transfer function in each of its modes to conform to that defined by the corresponding electronic filter. The design process requires a certain amount of mathematical modelling in order to specify hardware requirements, assess control loop stabilities and predict improvements in suspension performance. Air-Springs, commonly used on modern Railway Rolling Stock, can be used to give an active suspension by controlling the air pressure.