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.