Railway track (CwthE and UIC terminology) or railroad track (NAmE), also known as permanent way (per way) (CwthE) or "P way" (BrE and Indian English), is the structure on a railway consisting of the rails, fasteners, sleepers (railroad ties in American English) and ballast (or slab track), plus the underlying subgrade. It enables trains to move by providing a dependable, low-friction surface on which steel wheels can roll. Early tracks were constructed with wooden or cast-iron rails and wooden or stone sleepers. Since the 1870s, rails have almost universally been made from steel.
Historical development
The first railway in Britain was the Wollaton Wagonway, built in 1603 between Wollaton and Strelley in Nottinghamshire. It used wooden rails and was the first of about 50 wooden-railed tramways built over the subsequent 164 years. These early wooden tramways typically used rails of oak or beech, attached to wooden sleepers with iron or wooden nails. Gravel or small stones were packed around the sleepers to hold them in place and provide a walkway for the people or horses that moved wagons along the track. The rails were usually about 3 feet (0.91 m) long and were not joined - instead, adjacent rails were laid on a common sleeper. The straight rails could be angled at these joints to form a primitive curved track. The first iron rails laid in Britain were at the Darby Ironworks in Coalbrookdale in 1767. When steam locomotives were introduced, starting in 1804, the track then in use proved too weak to carry the additional weight. Richard Trevithick's pioneering locomotive at Pen-y-darren broke the plateway track and had to be withdrawn. As locomotives became more widespread in the 1810s and 1820s, engineers built rigid track formations, with iron rails mounted on stone sleepers, and cast-iron chairs holding them in place. This proved to be a mistake, and was soon replaced with flexible track structures that allowed a degree of elastic movement as trains passed over them.
Structure
Traditional track structure
Traditionally, tracks are constructed using flat-bottomed steel rails laid on and spiked or screwed into timber or pre-stressed concrete sleepers (known as ties in North America), with crushed stone ballast placed beneath and around the sleepers. Most modern railroads with heavy traffic use continuously welded rails that are attached to the sleepers with base plates that spread the load. When concrete sleepers are used, a plastic or rubber pad is usually placed between the rail and the tie plate. Rail is usually attached to the sleeper with resilient fastenings, although cut spikes are widely used in North America. For much of the 20th century, rail tracks used softwood timber sleepers and jointed rails, and a considerable amount of this track remains on secondary and tertiary routes. In North America and Australia, flat-bottomed rails were typically fastened to the sleepers with dog spikes through a flat tie plate. In Britain and Ireland, bullhead rails were carried in cast-iron chairs which were spiked to the sleepers. In 1936, the London, Midland and Scottish Railway pioneered the conversion to flat-bottomed rail in Britain, though earlier lines had used it to some extent. Jointed rails were used at first because contemporary technology did not offer any alternative. However, the ballast's intrinsic weakness in resisting vertical loading causes it to become depressed, and a heavy maintenance workload is required to prevent unacceptable geometric defects at the joints. The joints also needed to be lubricated, and wear at the fishplate (joint bar) mating surfaces needed to be rectified by shimming. For this reason, jointed track is not financially appropriate for heavily operated railroads. Timber sleepers are made of many available timbers, and are often treated with creosote, chromated copper arsenate, or other wood preservatives. Pre-stressed concrete sleepers are often used where timber is scarce and where tonnage or speeds are high. Steel is used in some applications. Track ballast is usually stone crushed to particular specifications. Its purpose is to support the sleepers and allow some adjustment of their position while allowing free drainage.
Ballastless track
A disadvantage of traditional track structures is the high maintenance demand, particularly for surfacing (tamping) and lining, to restore the desired track geometry and smoothness of vehicle running. Weaknesses of the subgrade and drainage deficiencies also lead to heavy maintenance costs. This can be overcome by using ballastless track. In its simplest form, this consists of a continuous slab of concrete (like a highway structure) with the rails supported directly on its upper surface (using a resilient pad). There are many proprietary systems; variations include a continuously reinforced concrete slab and precast prestressed concrete units laid on a base layer. Many design permutations have been put forward. However, ballastless track has a high initial cost, and for existing railroads, upgrading to it requires closing the route for an extended period. Its whole-life cost can be lower due to reduced maintenance. Ballastless track is usually considered for new very high-speed or very high-loading routes, in short extensions that require additional strength (e.g., railway stations), or for localised replacement where there are exceptional maintenance difficulties, for example, in tunnels. Most rapid transit lines and rubber-tyred metro systems use ballastless track.
Continuous longitudinally supported track
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