A railroad tie or sleeper is an oblong object used as a base for railroad tracks. Traditionally, ties have been made of wood, often heavily creosoted, but steel has also been used and concrete is now widely used.
Ties are laid on top of sand, gravel or heavy crushed stone - called ballast. They are laid across the grade at intervals of about two feet. The steel rails are then laid atop the ties, perpendicular to them. If the ties are wood, then cleats are laid down and spikes driven through them into the ties to clamp down the rails. For concrete ties, steel clips (for example the Pandrol® Clip) are often used to fasten the rails. After this is done, additional ballast is then added to fill the spaces between and around the ties to anchor them in place.
The ties then act as anchors and spacers for the rails, while providing a slight amount of give to accommodate weather and settling. The ties are "floating" in the top of the ballast. Failure of a single tie is generally insignificant to the usability and safety of the rails.
In recent years, wooden railroad ties have also become very popular for gardening and landscaping, both in creating retaining walls and raised-bed gardens, and sometimes for building steps as well. Traditionally, the ties sold for this purpose are old ones taken from rail lines when replaced with new ties, and their lifespan is often limited due to rot. Some entrepreneurs sell new ties. Unfortunately, due to presence of wood preservatives such as coal tar creosote and salts of heavy metals, railroad ties introduce an extra element of soil pollution into gardens and are avoided by many property owners.