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== Rails ==
[[Image:Rail stack.jpg|thumb|right|250px|Flat-bottom rails showing their cross-sectional shape.]]
[[Image:Reversible rail.jpg|thumb|right|250px|A reversible rail now used as a post.]]
Rails are usually made of steel, and in cross-section are usually shaped roughly like the capital letter "I", except that the top part (the "head") is much thicker than the bottom part.
These rails are known as ''flat-bottom'' rails. In the United States this type of rail is called "T" rail.

Another type of rail is symmetrical in cross-section, or nearly so, with two "heads".
Originally, this "reversible" rail was to allow the rail to be turned over and used again, but deterioration where the bottom "head" sat on the sleeper plate put an end to this practice.
However, similar rails, known as ''bullhead'' rails, continued to be made and used, mainly in Britain, which used the same fittings as the reversible rails.

Yet another type of rail is [[tram]]way rail, which is similar to flat-bottom rail but with a larger head containing a groove for the flange of the wheel.  These rails are designed to be laid flush with the surface of roads with the groove for the wheel flange contained in the head of the rail. This type of rail is called "girder" rail in the United States.

Rails are made in various sizes.
In America, rails are described in imperial units, whilst in Europe they are described in metric units.
Australia has switched from using imperial units to metric units.
Depending on the use of imperial or metric units, rail sizes are usually expressed in terms of pounds per yard or kilograms per metre.
Coincidentally, the pounds-per-yard figure is almost exactly double the kilograms-per-metre figure, making rough conversions easy.<!-- The last two sentences are copied from MY edit to a Wikipedia article.  -->