The flexible interface between the vehicle body and its bogies. Like primary suspension, it can be steel or rubber and is now, in addition, often composed of an air cushion system employing rubber bags or air bags. Originally, steel springs were used. In UK practice, they were mounted on a transverse beam known as a spring plank. Guides and shock absorbers were strategically placed to ensure stability. Such systems can still often be seen today and there are many variations on the theme.