Fishbelly rail

The rail profile is the cross sectional shape of a railway rail, perpendicular to the length of the rail.

Early rails were made of wood, cast iron or wrought iron. All modern rails are hot rolled steel of a specific cross sectional profile. Typically the cross section (profile) approximates an I-beam but is asymmetric about a horizontal axis (however see grooved rail below). The head is profiled to resist wear and to give a good ride, the foot is 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 trackwork, the heavier and faster the trains these tracks can carry.

The 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 branchline, siding or yard.

The weight of a rail per length is an important factor in determining rails strength and hence axleloads and speeds.

Weights are measured in pounds per yard or kilograms per metre; the pounds-per-yard figure is almost exactly double the kilograms-per-metre figure. Rails in Canada, the United Kingdom and United States are described using imperial units. In Australia, metric units are used as in mainland Europe.

Commonly, in rail terminology Pound is a contraction of the expression pounds per yard and hence a 132–pound rail means a rail of 132 pounds per yard.

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Wikipedia