Track (rail transport)


Track (rail transport)
Twin rail tracks in a wooded area
Track gauge
by size
Broad gauge
Standard gauge
Narrow gauge
Minimum gauge
List of rail gauges

Break-of-gauge
Dual gauge
Gauge conversion
Track
Tramway track

The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast (or slab track), plus the underlying subgrade. For clarity it is often referred to as railway track (British English and UIC terminology) or railroad track (predominantly in the United States).

The term permanent way also refers to the track in addition to lineside structures such as fences etc.

Contents

Track structure

Traditional track structure

Railroad tracks on traditional wooden sleepers

Notwithstanding modern technical developments, the overwhelmingly dominant track form worldwide consists of flat-bottom steel rails supported on timber or pre-stressed concrete sleepers (railroad ties in the US), which are themselves laid on crushed stone ballast.

Most railroads with heavy traffic use continuously welded rails supported by sleepers (ties) attached via baseplates which spread the load. A plastic or rubber pad is usually placed between the rail and the tieplate where concrete sleepers (ties) are used. The rail is usually held down to the sleeper (tie) with resilient fastenings, although cut spikes are widely used in North American practice.

Timber sleepers (ties) are of many available timbers, and are often treated with creosote, copper-chrome-arsenic, or other wood preservative. Pre-stressed concrete sleepers (ties) are often used where timber is scarce and where tonnage or speeds are high. Steel is used in some applications.

On this Japanese high speed line mats have been added to stabilize the ballast

The track ballast is customarily crushed stone, and the purpose of this is to support the ties and allow some adjustment of their position, while allowing free drainage.

Ballastless track

China high speed rail ballastless tracks

A disadvantage of traditional track structures is the heavy demand for maintenance, particularly surfacing (tamping) and lining to restore the desired track geometry and smoothness of vehicle running. Weakness of the subgrade and drainage deficiencies also lead to heavy maintenance costs. This can be overcome by using ballastless track. In its simplest form this consists of a continuous slab of concrete (like a highway structure) with the rails supported directly on its upper surface (using a resilient pad).

There are a number of proprietary systems, and variations include continuous in situ placing of a reinforced concrete slab, or alternatively the use of pre-cast pre-stressed concrete units laid on a base layer. Many permutations of design have been put forward.

However ballastless track is very expensive in first cost, and in the case of existing railroads requires closure of the route for a somewhat long period. Its whole life cost can be lower because of the great reduction in maintenance requirement. Ballastless track is usually considered for new very high speed or very high loading routes, in short extensions that require additional strength (i.e. rail station), or for localised replacement in the case of exceptional maintenance difficulties.

Ladder track

Ladder track utilizes longitudinal sleepers with gauge restraining cross members, it can be considered a development of Baulk road. Both ballasted and ballastless types exist.

Obsolete track types

For much of the 20th century, rail track used softwood timber ties and jointed rails, and considerable extents of this track type remains on secondary and tertiary routes. The rails were typically of flat bottom section fastened to the ties with dogspikes through a flat tieplate in North America and Australia, and typically of bullhead section carried in cast iron chairs in British and Irish practice.

Jointed rails were used, at first because the technology did not offer any alternative. However the intrinsic weakness in resisting vertical loading results in the ballast support becoming depressed and a heavy maintenance workload is imposed to prevent unacceptable geometrical defects at the joints. The joints also required to be lubricated, and wear at the fishplate (joint bar) mating surfaces needed to be rectified by shimming. For this reason jointed track is not financially appropriate for heavily operated railroads.

Historical development

The technology of rail tracks developed over a long period, starting with primitive timber rails in mines in the 17th century. This is fully discussed in the article Permanent way.

Rail

Cross-sections of flat-bottomed rail, which can rest directly on the sleepers, and bullhead rail which sits in a chair (not shown)

Hot rolled steel in the profile (cross section) of an asymmetrical I-beam is usually used as the surface on which railway wheels run.[1] Unlike some other uses of iron and steel, railway rails are subject to very high stresses and have to be made of very high-quality steel alloy. It took many decades to improve the quality of the materials, including the change from iron to steel. The heavier the rails and the rest of the trackwork, the heavier and faster the trains the track can carry.

Profiles of rail include:

  • Bullhead rail
  • Grooved rail
  • Vignoles rail (flat-bottomed rail)
  • Flanged T rail
  • Bridge rail (inverted U)
  • Barlow rail (inverted V)

North American railroads until the mid- to late-20th century used rails 39 ft (11.89 m) long so they could be carried to and from a worksite in gondola cars (open wagons), often 40 ft (12 m) long; as gondola sizes increased, so did rail lengths.

The world's longest rail sections are 120m long and are made by various companies.[2],[3],[4]

Rail classification (weight)

Weight mark on a jointed segment of 155 lb/yd (76.9 kg/m) "Pennsylvania Special" rail, the heaviest grade of rail to be mass-produced

Rail is graded by weight over a standard length. Heavier rail can support greater axle loads and higher train speeds without sustaining damage than lighter rail, but at a greater cost. In North America and the UK, rail is graded in pounds per yard (usually shown as pound or lb), so 130-pound rail would weigh 130 lb/yd (64.5 kg/m). The usual range is 115 to 141 lb/yd (57.0 to 69.9 kg/m). In Europe, rail is graded in kg/m and the usual range is 40 to 60 kg/m (80.6 to 121.0 lb/yd). The heaviest rail mass-produced was 155 pounds per yard (76.9 kg/m) and was rolled for the Pennsylvania Railroad. The UK is in the process of transition from the imperial to metric rating of rail.

Joining rails

Rails are produced in fixed lengths and need to be joined end-to-end to make a continuous surface on which trains may run. The traditional method of joining the rails is to bolt them together using metal fishplates, producing jointed track. For more modern usage, particularly where higher speeds are required, the lengths of rail may be welded together to form continuous welded rail (CWR).

Jointed track

Fishplate between two sections of jointed bullhead rail
Bonded main line 6-bolt rail joint on a segment of 155 lb/yd (76.9 kg/m) rail. Note how bolts are oppositely oriented to prevent complete separation of the joint in the event of being struck by a wheel during a derailment.

Jointed track is made using lengths of rail, usually around 20 m (66 ft) long (in the UK) and 39 or 78 feet (11.9 or 23.8 m) long (in North America), bolted together using perforated steel plates known as fishplates (UK) or joint bars (North America).

Fishplates are usually 600 mm (1.97 ft) long, used in pairs either side of the rail ends and bolted together (usually four, but sometimes six bolts per joint). The bolts may be oppositely-oriented so that in the event of a derailment and a wheel flange striking the joint, only some of the bolts will be sheared, reducing the likelihood of the rails misaligning with each other and exacerbating the seriousness of the derailment. (This technique is not applied universally, British practice being to have all the bolt heads on the same side of the rail.) Small gaps known as expansion joints are deliberately left between the rail ends to allow for expansion of the rails in hot weather. The holes through which the fishplate bolts pass are oval to allow for movement with expansion.

British practice was to have the rail joints on both rails adjacent to each other, while North 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" sound. Unless it is well-maintained, jointed track does not have the ride quality of welded rail and is less desirable for high speed trains. However, jointed track is still used in many countries on lower speed lines and sidings, and is used extensively in poorer countries due to the lower construction cost and the simpler equipment required for its installation and maintenance.

A major problem of jointed track is cracking around the bolt holes, which can lead to the rail head (the running surface) breaking. This was the cause of the Hither Green rail crash which caused British Railways to begin converting much of its track to Continuous Welded Rail.

Insulated joints

Where track circuits exist for signalling purposes, insulated block joints are required. These compound the weaknesses of ordinary joints. Specially-made glued joints, where all the gaps are filled with epoxy resin, increase the strength again.

As an alternative to the insulated joint, audio frequency track circuits can be employed using a tuned loop formed in approximately 20 m of the rail as part of the blocking circuit. Another alternative is the axle counter, which can reduce the number of track circuits and thus the number of insulated rail joints required.

Continuous welded rail

Welded rail joint
A pull-apart on the Long Island Rail Road Babylon Branch being repaired by using flaming rope to expand the rail back to a point where it can be joined together

Most modern railways use continuous welded rail (CWR), sometimes referred to as ribbon rails. In this form of track, the rails are welded together by utilising flash butt welding to form one continuous rail that may be several kilometres long, or thermite welding to repair or splice together existing CWR segments. Because there are few joints, this form of track is very strong, gives a smooth ride, and needs less maintenance; trains can travel on it at higher speeds and with less friction. Welded rails are more expensive to lay than jointed tracks, but have much lower maintenance costs. The first welded track was used in Germany in 1924 and the US in 1930[5] and has become common on main lines since the 1950s.

Flash butt welding is the preferred process which involves an automated track-laying machine running a strong electrical current through the touching ends of two unjoined pieces of rail. The ends become white hot due to electrical resistance and are then pressed together forming a strong weld. Thermite welding is a manual process requiring a reaction crucible and form to contain the molten iron. Thermite-bonded joints are also seen as less reliable and more prone to fracture or break.

If not restrained, rails would lengthen in hot weather and shrink in cold weather. To provide this restraint, the rail is prevented from moving in relation to the sleeper by use of clips or anchors. Anchors are more common for wooden sleepers, whereas most concrete or steel sleepers are fastened to the rail by special clips which resist longitudinal movement of the rail. There is no theoretical limit to how long a welded rail can be. However, if longitudinal and lateral restraint are insufficient, the track could become distorted in hot weather and cause a derailment. Distortion due to heat expansion is known in North America as sun kink, and elsewhere as buckling. In North America a rail broken due to cold-related contraction is known as a pull-apart. Attention needs to be paid to compacting the ballast effectively, including under, between, and at the ends of the sleepers, to prevent the sleepers from moving. In extreme hot weather special inspections are required to monitor sections of track known to be problematic.

After new segments of rail are laid, or defective rails replaced (welded-in), the rails can be artificially stressed if the temperature of the rail during laying is different than what is desired. The stressing process involves either heating the rails causing them to expand,[6] or stretching the rails with hydraulic equipment. They are then fastened (clipped) to the sleepers in their expanded form. This process ensures that the rail will not expand much further in subsequent hot weather. In cold weather the rails try to contract, but because they are firmly fastened, cannot do so. In effect, stressed rails are a bit like a piece of stretched elastic firmly fastened down.

CWR rail is laid (including fastening) at a temperature roughly midway between the extremes experienced at that location (this is known as the "rail neutral temperature"). This installation procedure, along with normal track structure strength, is intended to prevent tracks from buckling in summer heat or pulling apart in winter cold. In North America, because broken rails are typically detected by the signaling system; they are seen as less of a problem than heat kinks which are not detected.

An expansion joint on the Cornish Main Line, England

Joints are used in continuous welded rail when necessary, usually for signal circuit gaps. Instead of a joint that passes straight across the rail, the two rail ends are sometimes cut at an angle to give a smoother transition. In extreme cases, such as at the end of long bridges, a breather switch (referred to in North America and Britain as an expansion joint) gives a smooth path for the wheel while allowing the end of one rail to expand in relation to the next rail.

Rail support (sleeper/tie)

A railroad tie (also called a cross-tie in North American usage, or a railway sleeper outside North America) is a rectangular object on which the rails are supported and fixed. The tie has two main roles: to transfer the loads from the rails to the track ballast and the ground underneath, and to hold the rails to the correct width apart (to maintain the rail gauge). They are generally laid transverse (perpendicular) to the rails.

Fixing rails to railroad ties

Various methods exist for fixing the rail to the sleeper (railroad tie). Historically spikes gave way to cast iron chairs fixed to the sleeper, more recently springs (such as Pandrol clips) are used to fix the rail to the sleeper chair.

Portable track

Sometimes rail tracks are designed to be portable and moved from one place to another as required.

On the Panama Canal, tracks were so lifted around as is seen in moving pictures taken of the excavation works. These tracks were standard gauge, and the rolling stock full size.

Cane railways often had permanent tracks for the main lines, with portable tracks serving the canefields themselves. These tracks were narrow gauge (for example, 2 ft  (610 mm)) and the portable track came in straights, curves and turnouts rather like on a model railway.[7]

Decauville was a source of many portable light rail tracks, also used for military purposes.

The permanent way is so called because temporary way tracks were often used in the construction of that permanent way.

Gauge

Measuring rail gauge

During the early days of rail, there was considerable variation in the gauge used by different systems. Today, 60% of the world's railways use a gauge of 1,435 mm (4 ft 8 12 in), known as standard or international gauge.[citation needed] Gauges wider than standard gauge are called broad gauge; narrower, narrow gauge. Some stretches of track are dual gauge, with three (or sometimes four) parallel rails in place of the usual two, to allow trains of two different gauges to use the same track.[8]

Gauge can safely vary over a range. For example, U.S. federal safety standards allow standard gauge to vary from 4 ft 8 in (1,422 mm) to 4 ft 9 12 in (1,460 mm) for operation up to 60 mph (96.6 km/h).

Track maintenance

Maintenance of way equipment in Italy

Track needs regular maintenance to remain in good order, especially when high-speed trains are involved. Inadequate maintenance may lead to a "slow order" (North American terminology, a "slack" or speed restriction in the United Kingdom) being imposed to avoid accidents (see Slow zone). Track maintenance was at one time hard manual labour, requiring teams of labourers (US: gandy dancers, UK: platelayers or trackmen, Australia: fettlers), who used lining bars to correct irregularities in horizontal alignment (line) of the track, and tamping and jacks to correct vertical irregularities (surface). Currently, maintenance is facilitated by a variety of specialised machines.

A tie replacement train in Pennsylvania

The surface of the head of each of the two rails can be maintained by using a railgrinder.

Common maintenance jobs include changing crossties (sleepers), lubricating and adjusting switches, tightening loose track components, and surfacing and lining track to keep straight sections straight and curves within maintenance limits.

Spraying ballast with herbicide to prevent weeds growing through and disrupting the ballast is typically done with a special weed killing train.

Rail tracks with vegetation
Flange oilers lubricate wheel flanges to reduce rail wear in tight curves, Middelburg, Mpumalanga, South Africa

Over time, ballast is crushed or moved by the weight of trains passing over it, periodically requiring relevelling ("tamping") and eventually to be cleaned or replaced. If this is not done, the tracks may become uneven causing swaying, rough riding and possibly derailments. An alternative to tamping is to lift the rails and sleepers and reinsert the ballast beneath. For this, specialist "stone blower" trains are used.

Rail inspections utilize nondestructive testing methods to detect internal flaws in the rails. This is done by using specially equipped HiRail trucks, inspection cars, or in some cases handheld inspection devices.

Rails must be replaced before the railhead profile wears to a degree that may trigger a derailment. Worn mainline rails usually have sufficient life to be used on a branch line, siding or stub afterwards and are "cascaded" to those applications.

The environmental conditions along railroad track create a unique railway ecosystem. This is particularly so in the United Kingdom where steam locomotives are only used on special services and vegetation has not been trimmed back so thoroughly. This creates a fire risk in prolonged dry weather.

In the UK, the cess is used by track repair crews to walk to a work site, and as a safe place to stand when a train is passing. This helps when doing minor work, while needing to keep trains running, by not needing a Hi-railer or transport vehicle blocking the line to transport crew to get to the site.

Track bed and foundation

Railway tracks are generally laid on a bed of stone track ballast or track bed, in turn is supported by prepared earthworks known as the track formation. The formation comprises the subgrade and a layer of sand or stone dust (often sandwiched in impervious plastic), known as the blanket, which restricts the upward migration of wet clay or silt. The may also be layers of waterproof fabric to prevent water penetrating to the subgrade. The track and ballast form the permanent way. The term foundation may be used to refer to the ballast and formation, i.e. all man-made structures below the tracks.

Section through railway track and foundation showing the ballast and formation layers.
The layers are slightly sloped to help drainage.

Additional measures are required where the track is laid over permafrost, such as on the Qingzang Railway in Tibet. For example, transverse pipes through the subgrade allow cold air to penetrate the formation and prevent that subgrade from melting.

The sub-grade layers are slightly sloped to one side to help drainage of water. Rubber sheets may be inserted to help drainage and also protect iron bridgework from being affected by rust.

See also

References

Further reading

  • Pike, J., (2001), Track, Sutton Publishing, ISBN 0-7509-2692-9
  • Firuziaan, M. and Estorff, O., (2002), Simulation of the Dynamic Behavior of Bedding-Foundation-Soil in the Time Domain, Springer Verlag.
  • Robinson, A M (2009). Fatigue in railway infrastructure. Woodhead Publishing Limited. ISBN 9781855737402. 
  • Lewis, R (2009). Wheel/rail interface handbook. Woodhead Publishing Limited. ISBN 9781845694128. 

External links


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