Rail tracks

From Academic Kids

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image:Rail_track.jpg

Railroad or railway tracks are used on railways, which, together with railroad switches (points), guide trains without the need for steering. Tracks consist of two parallel steel rails, which are laid and fastened upon sleepers (or cross ties) which are embedded in ballast to form the railroad track.

Rails, being made of steel, can carry heavier loads than any other material.

Sleepers spread the load from the rails over the ground, and also serve to hold the rails a fixed distance apart (called the gauge).

Rail tracks are normally laid on a bed of coarse stone chippings known as ballast, which combines resilience, some amount of flexibility, and good drainage; however, track can also be laid on or into concrete (this is called slab track).

Across bridges track is often laid on sleepers across longitudinal timbers.

Contents

Railway rail

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Cross-sections of flat-bottomed and bullhead rails.

Unlike other uses of iron and steel, railway rails are subject to very high stresses and have to be 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 pose no problems with, say, reinforcing rods for buildings, can lead to broken rails and dangerous derailments when used on railway tracks.

The rails represent a substantial fraction of the cost of a railway line.

Railways rails are made in a large number of different sizes (weights), and the choice of size is a balance of economic and technical factors.

Some common european rail sizes include:

  • 40 kg/m (81 lb/yd)
  • 50 kg/m (101 lb/yd)
  • 60 kg/m (121 lb/yd)

Some common U.S. rail sizes include:

  • 115 lb/yd (57 kg/m)
  • 133 lb/yd (66 kg/m)
  • 136 lb/yd (67 kg/m)
  • 140 lb/yd (69 kg/m)

Rails in the United Kingdom and United States are still described using imperial units. The examples in the diagram opposite are 113 and 95 pounds per yard (56 kg/m and 47 kg/m)respectively.


Jointed track

There are different ways of joining rails together to form tracks. The traditional way of doing this, was to bolt rails together in what is known as jointed track. In this form of track, lengths of rail, usually around 20 metres (60 feet) long are laid and fixed to sleepers (UK) (crossties, or simply ties in US practice), and are joined to other lengths of rail with steel plates known as fishplates (UK) or joint bars (US).

Fishplates or joint bars are usually 60 centimetres (2 feet) long, and are bolted through each side of the rail ends with bolts (usually four, but sometimes up to six). Small gaps are deliberately left between the rails, which are known as "expansion joints" to allow for expansion of the rails in hot weather, the holes through which the fishplate bolts pass are oval to allow for expansion.

British practice was always to have the rail joints on both rails at the same place, while American practice is to stagger them.

Because of the small gaps left between the rails, when trains pass over jointed tracks they make a "clickety clack, clickety clack" noise. Unless it is very well maintained, jointed track gives a fairly bumpy and uncomfortable ride, and is unsuitable for high speed trains because it is too weak. However it is still used in many countries on lower speed lines, unimportant lines, and sidings. Most railroad track in the United States is still of this type, however, and laid on timber ties; the lower speeds of American railroads make the disadvantages less apparent and the abundant supply of timber in the US makes its use for railroad ties much cheaper than in Europe.

Jointed track is still extensively used in poor countries, due to the cheaper construction costs and lack of modernisation of their railway systems. Early railroads sometimes used strap-iron rails, which consisted of thin strips of iron strapped onto wooden rails. These rails were too fragile to carry heavy loads, but because the initial construction cost was less, this method was sometimes used to quickly build an inexpensive rail line. However, the long term expense involved in frequent maintenance outweighed any savings.

Continuous welded rail

Most modern railways use continuous welded rail (CWR); in this form of track the rails are welded together for several kilometres, to form one long continuous rail. Because there are few joints, this form of track is very strong and gives a smooth ride, and also needs less maintenance.

Because of its strength, trains travelling on welded track can travel at higher speeds and with less friction. Welded rails are more expensive to lay than jointed tracks, but are significantly cheaper to maintain.

As mentioned earlier, rails expand in hot weather and shrink in cold weather. Because welded track has very few expansion joints, if no special measures are taken, it could become distorted in hot weather and cause a derailment.

To avoid this happening welded rails are nearly always laid on concrete sleepers, which are so heavy they hold the rails firmly in place, and with plenty of ballast to stop the sleepers moving. After new segments of rail are laid, or defective rails replaced (welded in), the rails are artificially stretched so they expand (this is called stressing), they are then fastened (clipped) to the sleepers in their expanded form. This ensures that the rail will not expand much further in subsequent hot weather. However if temperatures reach outside normal ranges (i.e. a hotter than usual summer), it can cause problems with welded rails.

Joints are used in continuously welded rail when necessary; instead of a joint that passes straight across the rail, producing a loud noise and shock when the wheels pass over it, two sections of rail are cut at a steep angle and put together with a gap between them (a breather switch). This gives a much smoother transition yet still provides some expansion room.

Methods of fixing rail to sleepers/ties

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Mechanical semaphore in Koscierzyna

There are several methods used to fasten rail to wooden sleepers / ties. In traditional British practice, cast metal chairs were screwed to the sleepers, which took a style of rail known as bullhead which was somewhat figure-8 in cross-section - wider at top and bottom (known as the head and foot respectively) and smaller in the middle (the web). Keys, which were wedges of wood or sprung steel were then driven in between chair and rail to hold it in place.

The idea behind bullhead rails, was that because both the top and bottom of the rails were the same shape, when one side of the rail became worn, the rail could be turned over to the unused side, thus extending the rail's lifespan. In practice, bullhead rails have a flat base (narrower than flat-bottomed rail) and the top part has curved edges which fit the profile of the train wheels.

Like most of the world, Britain now uses flat bottomed rail (Vignoles rail) which has become the worldwide standard type of rail, which as the name suggests, has a flat base and can stand upright without support. A flat bottomed rail has a cross-section like that of an upside-down 'T' and is usually held to the sleeper with a baseplate, a metal plate which is attached to the sleeper, although for cheap construction they can be laid directly onto the sleepers.

Modern sleepers can be made of reinforced concrete and pressed steel, with rubber pads inserted between the sleeper and rail. This is done for two reasons: to give a smoother ride, and to prevent the sleeper shorting the track circuit, a low voltage which is passed through the rails for signalling purposes. This is different from "traction current" which powers electric trains.

A variety of different types of heavy-duty clips are used to fasten the rails to the underlying baseplate, one common one being the Pandrol fastener, named after its maker, which is shaped like a sturdy, stubby paperclip.

American practice normally uses spikes, which are fundamentally very large nails with bent-over heads to clasp the flat-bottomed rail. These are cheaper and simpler to install but can loosen if the tie rots - much more easily than the British chair does. This is mitigated by using very large and solid ties and using rot-proofing preservative.

Track maintenance

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Broken railroad tracks in Alaska

Track needs frequent maintenance to remain in good order, the frequency increasing with higher-speed or heavier trains. This was formerly hard manual labour, including teams of gandy dancers who used levers to force rails back into place on steep turns, correcting the gradual shifting caused by the centrifugal force of passing trains. Currently, maintenance is facilitated by a variety of specialised machines.

The profile of the track is maintained using a railgrinder.

Common maintenance jobs include spraying ballast with weedkiller to prevent weeds growing through and disrupting the ballast. This is typically done with a special weedkilling train.

Over time, ballast is crushed by the weight of trains passing over it, and periodically it needs to be replaced. If this is not done then the tracks become uneven.

Broken or worn out rails also need replacing periodically. Mainline rails that get worn out, usually have life left in branchline use, and are "cascaded" to those branchlines.

History

It took many decades for weak and fragile iron rails to evolve into the strong and robust steel rails of today, and even then problems occur, such as happened with the Hatfield train derailment that happened in Great Britain on October 17, 2000 that involved gauge corner cracking.

The iron foundries making early rails often used some of the rails to build the tramways that bought iron ore and coal to those foundries.

Some early rails were made by William Jessop in the 1790's.

See also

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de:Gleis fa:ریل fr:Voie ferre ja:線路 sv:Rls zh:鐵路軌道 pl:tory

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