All modern railways use steel rails which are specifically rolled for the purpose from steel which has the required qualities of strength, fatigue endurance and wear and corrosion resistance. This type of steel is fully covered by British Standard Specification 11.
As has been mentioned previously, the shape of the rail has now become generally standardised as the Flat Bottom (FB) rail. This is sometimes known as the Vignoles rail, after the inventor. Main line railways in the UK have now standardised on the BS113A section rail for all important lines. The head of the rail has an almost flat top with curves at the outer edges designed to fit the shape of the wheel tyre. One of the features of a well matched rail head and wheel tyre is that, when the axis of the wheel 68 set coincides with the longitudinal axis of the track and the rail is set at its correct inclination of 1 in 20 to the vertical, the point of contact between the two is very close to the centre line of the rail. This is very desirable since it minimises the twisting effect on the rail which a concentrically applied wheel load would produce, and by keeping the contact area away from the gauge corner, reduces bot damage.
The rail head sides slope at 1 in 20. This is to compensate for the 1 in 20 inwards slope of the rails and not only makes it simpler to check the gauge but ensures that when side wear takes place the associated gauge widening is minimised.
The thick web of the BS113A section is designed to give the rail adequate shear strength to guard against fatigue failures, particularly around fishbolt holes and under heavy axle loads at joints. The foot of the rail is broad enough to give stability against roll-over, remembering that steering forces exerted by rolling stock produce torsional and lateral forces which have to be resisted by the rail and transmitted via the fastenings to the sleeper. In addition to the primary function, the rail has secondary functions relating to the carrying of track circuit currents and in some cases on electrified railways, conveying return traction currents.
Each section of rail that is used requires special steel castings, clips, bolts, resilient pads, fishplates, expansion switches and etc to make up the full structural system of the track. Most railway authorities endeavour to keep rail types and sizes to a minimum to ensure also that maintenance stocks of replacement components can also be kept to a sensible minimum. A great deal of capital can be tied up in stock which is kept in stores just to cover an eventuality which may never happen.
There are also a number of signal related track components, like block joints, which are incorporated into the track structural system.
With third and fourth rail DC electrification systems there are also a large number of insulators and other fittings relating to the track which are required.