A traditional track structure supports rails on wood crossties partially embedded in a ballast of crushed stone or other aggregate, resting on a subgrade of varying load capacity. At the same time that supplies of hardwood for ties have been depleted in some parts of the world, prestressed concrete technology has advanced and railroads have moved toward increased use of prestressed concrete ties. This is a trend that is only in its infancy in the United States.
Concrete ties may be one-piece prestressed concrete, or they may be made of two concrete end blocks connected by a steel cross-piece. Using a solid concrete slab for track support offers one way to reduce or eliminate much of the costly track maintenance. Continuously reinforced slabs are the most widely used cast-in-place option to date. Precast slabs--both conventionally reinforced and prestressed--are in service. Subbases for the slab tracks have been made of crushed stone, cement-treated materials, asphalt-treated materials, lean concrete and expanded polystyrene concrete.