Also known as air springs, many modern passenger vehicles now employ a form of air suspension. The car body rests on a set of fthe rubber air cushions or bags, two being arranged on each bogie bolster. Compressed air is fed into the bags under the control of a levelling valve to ensure the correct pressure is maintained. Air bags are usually designed with a solid rubber core upon which the body will rest in the event of a burst bag. Normally the train can continue to run in this condition, usually at reduced speed. Air bags are also designed to allow for shear during transition through curves.