The transverse track structure element, usually of treated wood, which supports the rails and holds them at the proper gauge. It is in turn supported by the ballast. In addition to supporting the rails, wooden ties provide some measure of vibration and shock absorption and in signaled track, insulate the rails from each other. In a mile of conventional main line track there are 3250 wooden ties, each seven by nine inches in diameter and eight to nine feet long. Larger ties are used in turnouts and on bridges. Ties can also be made of steel or concrete.