Body weight supported treadmill training is an approach to gait rehabilitation following a stroke or spinal cord injury. Although lateral control of balance is an important aspect of walking, many of the currently available body weight support systems have a fixed pulley configuration which can lead to lateral forces being developed in the supporting cables, interfering with the lateral balance task. In this paper, a novel extension for body weight support systems, used for treadmill walking, is presented which features a system of pulleys and trolleys. A model is developed for the device along with a basic feedback controller in order to enable simulation of the concept. The lateral forces induced by the novel system are greatly reduced in comparison to a fixed pulley system. This device has applications in balance training within gait rehabilitation programs.