An image display system of the type which may be used to present video images to occupants of a vehicle (10), such as a subway train, by briefly illuminating individual images provided on display boards (14) in a tunnel or the like, in sequence, so that occupants of the vehicle (10) observe a motion picture. One aspect of the invention provides a control unit for an image display system including a series of images (16) positioned along the path of a vehicle and lighting for briefly illuminating individual images and a detector (20) arranged to output repeated measurements of the speed of a passing vehicle. The control unit comprises a processing means including a first system arranged to process the repeated speed measurements to produce an instantaneous estimate of the position of the vehicle along the vehicle path, and a second system arranged to derive the illumination timings from the instantaneous estimate of the position of the vehicle.