The rail profile is the cross sectional shape of a railway rail, perpendicular to its length.
Early rails were made of wood, cast iron or wrought iron. All modern rails are hot rolled steel with a cross section (profile) approximate to an I-beam, but asymmetric about a horizontal axis (however see grooved rail below). The head is profiled to resist wear and to give a good ride, and the foot profiled to suit the fixing system.
Unlike some other uses of iron and steel, railway rails are subject to very high stresses and are made of very high quality steel. It took many decades to improve the quality of the materials, including the change from iron to steel. Minor flaws in the steel that may pose no problems in other applications can lead to broken rails and dangerous derailments when used on railway tracks.
By and large, the heavier the rails and the rest of the track work, the heavier and faster the trains these tracks can carry.
Rails represent a substantial fraction of the cost of a railway line. Only a small number of rail sizes are made by steelworks at one time, so a railway must choose the nearest suitable size. Worn, heavy rail from a mainline is often reclaimed and downgraded for re-use on a branch line, siding or yard.
History
Early rails were used on horse drawn wagonways, originally with wooden rails, but from the 1760s using strap-iron rails, which consisted of thin strips of cast iron fixed onto wooden rails.<ref name="Bianculli2002">{ { cite book|last=Bianculli|first= Anthony J.|title=Trains and Technology: The American Railroad in the Nineteenth Century|publisher= University of Delaware Press|date= 2002|url=https://archive.org/details/trainstechnology0000bian|url-access=registration|page=85|chapter= Ch 5 From Strap Iron to High Iron|isbn=0-87413-802-7}}</ref> These rails were too fragile to carry heavy loads, but because the initial construction cost was less, this method was sometimes used to quickly build an inexpensive rail line. Strap rails sometimes separated from the wooden base and speared into the floor of the carriages above, creating what was referred to as a "snake head". However, the long-term expense involved in frequent maintenance outweighed any savings.
These were superseded by cast iron rails that were flanged (i.e. 'L' shaped) and with the wagon wheels flat. An early proponent of this design was Benjamin Outram of B. Outram & Co. which later became the Butterley Company in Ripley. His partner William Jessop preferred the use of "edge rails" in 1789 where the wheels were flanged and, over time, it was realised that this combination worked better. William Jessop's (fishbellied) "edge rails" were cast by the Butterley Company.
The earliest of these in general use were the so-called cast iron fishbelly rails from their shape. Rails made from cast iron were brittle and broke easily. They could only be made in short lengths which would soon become uneven. John Birkinshaw's 1820 patent, as rolling techniques improved, introduced wrought iron in longer lengths, replaced cast iron and contributed significantly to the explosive growth of railroads in the period 1825–40. The cross-section varied widely from one line to another, but were of three basic types as shown in the diagram. The parallel cross-section which developed in later years was referred to as bullhead.
Meanwhile, in May 1831, the first flanged T rail (also called T-section) arrived in America from Britain and was laid into the Pennsylvania Railroad by Camden and Amboy Railroad. They were also used by Charles Vignoles in Britain.
The first steel rails were made in 1857 by Robert Forester Mushet, who laid them at Derby station in England. Steel is a much stronger material, which steadily replaced iron for use on railway rail and allowed much longer lengths of rails to be rolled.