A levelling valve is used to determine the variations in vehicle weight caused by loading and unloading. It is a pneumatic device mounted on the vehicle underframe and connected to the bogie, which measures the body's vertical displacement relative to the bogie caused by weight changes. The valve sends a pneumatic signal to the traction and braking systems to adjust their rates according to the vehicle weight. It is used in conjunction with air suspension to cause the vehicle height to remain constant regardless of load. The levelling valve is a fairly recent development (in railway terms) over the last 15 years and is used as part of modern traction and braking control systems. Some trains can be heard expelling air from their suspension system levelling valves as passengers alight and the system adjusts.