II.

Rails

The precursors of modern railroads were the wagonways, or tramroads (a tram was originally a coal wagon), built in England as early as the 16th century to facilitate the hauling of coal, ore, or stone from mines or quarries to ports or waterways. Although the first wagonways consisted merely of parallel lines of planks, they enabled draft animals to achieve greater speeds and pull much heavier loads than was possible over the bare surfaces of rutted and often muddy roads. Crossties were introduced in early tramroads to hold the timbers that made up the tracks in place. The wooden tracks were soon improved by facing them with strips of iron, and iron wheels on the wagons came into use. In 1767 a British foundry produced the first cast-iron rails, which withstood heavy loads better than iron-faced timbers.

In 1811 a British coal-mine owner was granted a patent on a toothed rail to be traversed by toothed wheels. This rack-and-pinion principle is still applied in auxiliary third rails used in a few railroads, for example, on Pikes Peak in the United States and on some Swiss mountainsides, where cars must be pulled up extremely steep grades.

Modern rails evolved from the edge rails used in northern England at the beginning of the 19th century. Wagons were held on this type of track by flanges extending downward from the inner edges of the wheels. (Many authorities define railroads and railways, in distinction from tramroads, as lines on which the rails are raised above the roadbed.) After the practicability of the locomotive was demonstrated in 1829, and as locomotives replaced horses, mules, and the occasional stationary engines used to pull cars up grades by means of cables, edge rails came into general use.

Rails of various shapes were devised. The prototype of those used today throughout the world, except in Great Britain, was the flat-footed T rail designed in 1830 by the American inventor Robert Livingston Stevens, who was the chief engineer and president of the newly established Camden and Amboy Railroad in New Jersey. In this type of design the T-shaped rail stands on a base broader than the head of the T, forming flanges at each side that permit the rail to be spiked directly to the ties. In the United States today the rail is mounted on metal plates, called tie plates, which are wider than the rail's base and prevent it from cutting into the ties.

The bridge rail, which in cross section formed an inverted U and which fitted over longitudinal timbers, was used on the Great Western Railway in England until 1892. Standard in Britain today is the bullheaded rail, evolved from an I-shaped rail introduced in 1835. In theory the I rail (called a double-headed rail) could be reversed when the upper side became worn, but in practice this economy could not be effected, because the lower part of the rail also became worn by contact with the heavy metal braces, called chairs, that are required to hold the rail in an upright position. The bullheaded rail has a wider, thicker head than the I rail but also must be mounted in chairs, in which it is braced by wooden wedges.

A.

Wrought-Iron and Steel Rails

The first improvement on cast-iron rails were rails of wrought iron, introduced in 1820 in England, where the first steel rail was also manufactured. The manufacture of steel rails in the United States began in 1865, and they are now used throughout the world. Metallurgical advances in the 20th century greatly improved the quality of rail steel. Previously, transverse fissures or cracks often developed inside rails during use, until engineers discovered that the flaws from which these cracks spread were formed when rails hot from the rolling mill were cooling. All rails manufactured for use in the United States now undergo a process of controlled cooling and inspection to prevent such defects. Usually they are also hardened at the ends by heat treatment.

Heavier trains requiring stronger track resulted in much heavier rails. The iron rails used in early railroading weighed less than about 20 kg/m (about 40 lb/yd), and the steel rails used at the beginning of the 20th century in many cases were not heavier than about 30kg/m (about 60 lb/yd). In the 1930s rails weighing 50 kg/m (100 lb/yd) or more, or in some instances more than about 65 kg/m (about 130 lb/yd) were used. Rails manufactured today for main-line use may weigh as much as 75.5 to 77 kg/m (152 to 155 lb/yd).

B.

Joints

Because each joint is a relatively weak spot in a track, design engineers have reduced the number of joints by lengthening the rails. The customary length when locomotives were introduced was 0.9 m (3 ft), but in the 1830s this was increased to 4.6 or 6.1 m (15 or 20 ft). Early in the 20th century the most common length for rails was 9.1 m (30 ft), and this figure soon became 10 m (33 ft) when 12.2-m (40-ft) freight cars came into general use. To some extent the length of rails has been limited by difficulties in transporting them. Rails 18.3 m (60 ft) long, used on one British railroad as early as 1894, were installed on some United States railroads, others of which have 13.7-m (45-ft) rails.

In the United States rails are often butt-welded together to form lengths as long as 0.4 km (0.25 mi). At first this was done cautiously for fear that expansion and contraction due to temperature changes would cause buckling in great lengths of continuous rail. Experience showed, however, that longitudinal expansion and contraction are not excessive and need not lead to buckling. Techniques were developed for making butt welds as strong as the rails themselves. Where welding is not used, rails are joined by bars bolted to the sides so as to cover the joint. Stevens is credited with inventing the first such joint. On earlier railroads using metal rails, the individual sections were not fastened together in any way.

Advances in track construction in the 20th century included using longer and stronger joint bars and wider tie plates to spread the weight of trains more evenly on the ties. Tie plates with shoulders to brace the rail on either side are used, and nearly all U.S. railroads have special braces called anticreepers, designed to prevent longitudinal displacement.

Beginning in 1925 and continuing at an accelerated rate after that, especially after World War II (1939-1945), the installation of centralized traffic control (CTC) increased track capacity on many railroads and lessened or even eliminated the need for additional pairs of rails. In this system the switches and signals over many kilometers of track are controlled by a single train dispatcher who sits before a panel or switchboard in a control room. On this panel the location of each train is shown automatically on an illuminated diagram. Below the diagram are knobs that control each signal and levers that control each switch on the line. Many railroads began to remove extra main-line tracks after the installation of CTC.