Though rails of steel are standard today, iron and even wood found widespread use in the 19th century. Many early railroads were built of wooden rails capped with thin iron bars or “straps” to provide a smooth running surface for the wheels.
Steel rail is composed of iron, carbon, manganese, and silicon, and contains impurities such as phosphorous, sulphur, gases, and slag. The proportions of these substances may be altered to achieve different properties, such as increased resistance to wear on curves.
The standard configuration for North American rail resembles an upside down T. The three parts of T-rail are called the base
[ foot ], web, and head.

[Text Credit] ABCs of Railroading

Rail / Track Gauge:
The nominal track gauge is the distance between the inner faces of the rails. In current practice, it is specified at a certain distance below the rail head as the inner faces of the rail head (the gauge faces) are not necessarily vertical.
The US standard railroad gauge is 4 feet, 8.5 inches (Gauge means width between the two rails). The U.S. federal safety standards allow the standard gauge to vary from 4 ft 8 in (1,420 mm) to 4 ft 9 1/2 in (1,460 mm) for operation up to 60 mph (97 km/h).

Weight:
Rail weight is measured in linear yards. This weight per yard can be affected by the cross section (shape) of the rail and is usually between 112 and 145 pounds per yard.

Length:
At one time a common length for railroad cars was forty feet and because rail lengths were transported on railroad cars the standard manufactured length of the rail sections became thirty-nine feet - a standard still in effect today.

Connection:
Rails are usually joined together with standard joint (or angle) bars each of which has four or six bolts. ( There are other custom configurations of joint bars.) When the tracks are set in place a small gap is left between the track ends before the joint bar is installed. This is to allow for thermal expansion of the steel rails which, without this small gap, would expand into each other and deform under the pressure or possibly even buckle.

Note that the six bolts alternate in the d irection in which they face (U.S.). Note also the short jumper wire to maintain electrical continuity from one rail to the next. Welded Rail: From its early use on a handful of roads in the 1940’s, welded rail has come to be preferred for almost all applications. It is produced by welding standard 39-foot (or newer 78-foot) segments together into quarter-mile lengths at dedicated plants. The rails are transported to where they’re needed in special trains, which are pulled slowly out from under the rail when it is to be unloaded. When in place, CWR is often field-welded into even greater lengths.

Rail Inclination
The conical surface of the train wheel has the undesired effect of directing the weight of the car in a direction perpendicular to the wheel surface. This outward force tends to spread the gauge apart.

To minimize this effect the rails are inclined slightly towards the inside (gauge side). In the following image the "field" side is the outside of the track [ the side nearer to the surrounding fields ]. The gauge side is the side of the rail on the inside of the track.

The standard angle of inclination (Cl) in the U.S. is 1:40 which means that every 40 units of lateral extension at the base of the rail results in a rise of 1 unit. Trigonometry shows that the tangent (1/40) =0.025 and the resulting angle A) =1.43 degrees. One method of creating this inclination is to use rail plates which are beveled at the angle of inclination as shown here.