Safe Working Load of 6Kg. Made from Moulded PlasticEye Diameter: 6.5mm SWL: 6kG. The Doughty Rail Ball Raced Runners are a moulded plastic component. Each runner has a SWL of 6Kg and it is recommended these runners are at 300mm intervals throughout the span of the Curtain/Drape.. curtain track-doughty rail