Construction and maintenance of High Ways
Highway engineering is an engineering discipline branching from civil engineering that involves the planning, design, construction, operation, and maintenance of roads, bridges, and tunnels to ensure safe and effective transportation of people and goods. Highway engineering became prominent towards the latter half of the 20th Century after World War II. Standards of highway engineering are continuously being improved. Highway engineers must take into account future traffic flows, design of highway intersections/interchanges, geometric alignment and design, highway pavement materials and design, structural design of pavement thickness, and pavement maintenance.
The most appropriate location, alignment, and shape of a highway are selected during the design stage. Highway design involves the consideration of three major factors (human, vehicle, and roadway) and how these factors interact to provide a safe highway. Human factors include reaction time for braking and steering, visual acuity for traffic signs and signals, and car-following behaviour. Vehicle considerations include vehicle size and dynamics that are essential for determining lane width and maximum slopes, and for the selection of design vehicles. Highway engineers design road geometry to ensure stability of vehicles when negotiating curves and grades and to provide adequate sight distances for undertaking passing maneuvers along curves on two-lane, two-way roads.
Highway and transportation engineers must meet many safety, service, and performance standards when designing highways for certain site topography. Highway geometric design primarily refers to the visible elements of the highways. Highway engineers who design the geometry of highways must also consider environmental and social effects of the design on the surrounding infrastructure.
There are certain considerations that must be properly addressed in the design process to
successfully fit a highway to a site's topography and maintain its safety. Some of these design considerations include:
Design traffic volume
Number of lanes
Level of service
Alignment, super-elevation, and grades
Horizontal and vertical clearance
The operational performance of a highway can be seen through drivers’ reactions to the design considerations and their interaction.
The materials used for roadway construction have progressed with time, dating back to the early days of the Roman Empire. Advancements in methods with which these materials are characterized and applied to pavement structural design have accompanied this advancement in materials.
There are two major types of pavement surfaces - Portland Cement Concrete (PCC) and Hot-Mix Asphalt (HMA). Underneath this wearing course are material layers that give structural support for the pavement system. These underlying surfaces may include either the aggregate base and sub-base layers, or treated base and sub-base layers, and additionally the underlying natural or treated subgrade. These treated layers may be cement-treated, asphalt-treated, or lime-treated for additional support.
Types of Pavements
Flexible Pavement Design
A flexible, or asphalt, pavement typically consists of three or four layers. For a four layer flexible pavement, there is a surface course, base course, and sub-base course constructed over a compacted, natural soil subgrade. When building a three layer flexible pavement, the sub-base layer is not used and the base course is placed directly on the natural subgrade.
A flexible pavement's surface layer is constructed of hot-mix asphalt (HMA). Unstabilized aggregates are typically used for the base course; however, the base course could also be stabilized with asphalt, Portland cement, or another stabilizing agent. The sub-base is generally constructed from local aggregate material, while the top of the subgrade is often stabilized with cement or lime.
With flexible pavement, the highest stress occurs at the surface and the stress decreases as the depth of the pavement increases. Therefore, the highest quality material needs to be used for the surface, while lower quality materials can be used as the depth of the pavement increases. The term "flexible" is used because of the asphalt’s ability to bend and deform slightly, then return to its original position as each traffic load is applied and removed. It is possible for these small deformations to become permanent, which can lead to rutting in the wheel path over an extended time.
The service life of a flexible pavement is typically designed in the range of 15 to 20 years. Required thicknesses of each layer of a flexible pavement vary widely depending on the materials used, magnitude, number of repetitions of traffic loads, environmental conditions and the desired service life of the pavement. Factors such as these are taken into consideration during the design process so that the pavement will last for the designed life without excessive distresses.
Rigid Pavement Design
Rigid pavements are generally used in constructing airports and major highways. They commonly serve as heavy-duty industrial floor slabs, port and harbor yard pavements, and heavy-vehicle park or terminal pavements. Like flexible pavements, rigid highway pavements are designed as all-weather, long-lasting structures to serve modern day high-speed traffic. Offering high quality riding surfaces for safe vehicular travel, they function as structural layers to distribute vehicular wheel loads in such a manner that the induced stresses transmitted to the subgrade soil are of acceptable magnitudes.
Portland Cement Concrete (PCC) is the most common material used in the construction of rigid pavement slabs. The reason for its popularity is due to its availability and the economy. Rigid pavements must be designed to endure frequently repeated traffic loadings. The typical designed service life of a rigid pavement is between 30 and 40 years, lasting about twice as long as a flexible pavement.
One major design consideration of rigid pavements is reducing fatigue failure due to the repeated stresses of traffic. Fatigue failure is common among major roads because a typical highway will experience millions of wheel passes throughout its service life. In addition to design criteria such as traffic loadings, tensile stresses due to thermal energy must also be taken into consideration. As pavement design has progressed, many highway engineers have noted that thermally induced stresses in rigid pavements can be just as intense as those imposed by wheel loadings. Due to the relatively low tensile strength of concrete, thermal stresses are extremely important to the design considerations of rigid pavements.
Rigid pavements are generally constructed in three layers - a prepared subgrade, base or sub-base, and a concrete slab. The concrete slab is constructed according to a designed choice of plan dimensions for the slab panels, directly influencing the intensity of thermal stresses occurring within the pavement. In addition to the slab panels, temperature reinforcements must be designed to control cracking behavior in the slab. Joint spacing is determined by the slab panel dimensions.
Three main types of concrete pavements commonly used are Jointed Plain Concrete Pavement (JPCP), Jointed Reinforced Concrete Pavement (JRCP), and Continuously Reinforced Concrete Pavements (CRCP). JPCP’s are constructed with contraction joints which direct the natural cracking of the pavement. These pavements do not use any reinforcing steel. JRCP’s are constructed with both contraction joints and reinforcing steel to control the cracking of the pavement. High temperatures and moisture stresses within the pavement creates cracking, which the reinforcing steel holds tightly together. At transverse joints, dowel bars are typically placed to assist with transferring the load of the vehicle across the cracking. CRCP’s solely rely on continuous reinforcing steel to hold the pavement’s natural transverse cracks together. Pre-stressed concrete pavements have also been used in the construction of highways; however, they are not as common as the other three. Pre-stressed pavements allow for a thinner slab thickness by partly or wholly neutralizing thermally induced stresses or loadings.
Flexible Pavement Overlay Design
Over the service life of a flexible pavement, accumulated traffic loads may cause excessive rutting or cracking, inadequate ride quality, or an inadequate skid resistance. These problems can be avoided by adequately maintaining the pavement, but the solution usually has excessive maintenance costs, or the pavement may have an inadequate structural capacity for the projected traffic loads.
Throughout a highway’s life, its level of serviceability is closely monitored and maintained. One common method used to maintain a highway’s level of serviceability is to place an overlay on the pavement’s surface.
There are three general types of overlay used on flexible pavements: asphalt-concrete overlay, Portland cement concrete-overlay, and ultra-thin Portland cement concrete-overlay. The concrete layer in a conventional PCC overlay is placed un-bonded on top of the flexible surface. The typical thickness of an ultra-thin PCC overlay is 4 inches (100 cm) or less.
There are two main categories of flexible pavement overlay design procedures:
a) Component Analysis Design
Rigid Pavement Overlay Design
Near the end of a rigid pavement's service life, a decision must be made to either fully reconstruct the worn pavement, or construct an overlay layer. Considering an overlay can be constructed on a rigid pavement that has not reached the end of its service life, it is often more economically attractive to apply overlay layers more frequently. The required overlay thickness for a structurally sound rigid pavement is much smaller than for one that has reached the end of its service life. Rigid and flexible overlays are both used for rehabilitation of rigid pavements such as JPCP, JRCP, and CRCP. There are three subcategories of rigid pavement overlays that are organized depending on the bonding condition at the pavement overlay and existing slab interface.
Partially bonded overlays
Highway construction is generally preceded by detailed surveys and subgrade preparation. The methods and technology for constructing highways has evolved over time and become increasingly sophisticated. This advancement in technology has raised the level of skill sets required to manage highway construction projects. This skill varies from project to project, depending on factors such as the project's complexity and nature, the contrasts between new construction and reconstruction, and differences between urban region and rural region projects.
There are a number of elements of highway construction which can be broken up into technical and commercial elements of the system. Some examples of each are listed below:
Typically, construction begins at the lowest elevation of the site, regardless of the project type, and moves upward. By reviewing the geotechnical specifications of the project, information is given about:
Existing ground conditions
Required equipment for excavation, grading, and material transportation to and from the site
Properties of materials to be excavated
Dewatering requirements necessary for below-grade work
Shoring requirements for excavation protection
Water quantities for compaction and dust control
Sub-base Course Construction
A sub-base course is a layer designed of carefully selected materials that is located between the subgrade and base course of the pavement. The sub-base thickness is generally in the range of 4 to 16 inches, and it is designed to withstand the required structural capacity of the pavement section.
Common materials used for a highway sub-base include gravel, crushed stone, or subgrade soil that is stabilized with cement, fly ash, or lime. Permeable sub-base courses are becoming more prevalent because of their ability to drain infiltrating water from the surface. They also prevent subsurface water from reaching the pavement surface.
Base Course Construction
The base course is the region of the pavement section that is located directly under the surface course. If there is a sub-base course, the base course is constructed directly about this layer. Otherwise, it is built directly on top of the subgrade. Typical base course thickness ranges from 100 to 150 mm and is governed by underlying layer properties.
Heavy loads are continuously applied to pavement surfaces, and the base layer absorbs the majority of these stresses. Generally, the base course is constructed with an untreated crushed aggregate such as crushed stone, slag, or gravel. The base course material will have stability under the construction traffic and good drainage characteristics.
The base course materials are often treated with cement, bitumen, calcium chloride, sodium chloride, fly ash, or lime. These treatments provide improved support for heavy loads, frost susceptibility, and serves as a moisture barrier between the base and surface layers.
Surface Course Construction
There are two most commonly used types of pavement surfaces used in highway construction: hot-mix asphalt and Portland cement concrete. These pavement surface courses provide a smooth and safe riding surface, while simultaneously transferring the heavy traffic loads through the various base courses and into the underlying subgrade soils.
Hot-Mix Asphalt (HMA) Layers
Hot-mix asphalt surface courses are referred to as flexible pavements. The Super-pave System was developed in the late 1980s and has offered changes to the design approach, mix design, specifications, and quality testing of materials.
The construction of an effective, long-lasting asphalt pavement requires an experienced construction crew, committed to their work quality and equipment control.
Asphalt mix segregation
A prime coat is a low viscosity asphalt that is applied to the base course prior to laying the HMA surface course. This coat bonds loose material, creating a cohesive layer between the base course and asphalt surface.
A tack coat is a low viscosity asphalt emulsion that is used to create a bond between an existing pavement surface and new asphalt overlay. Tack coats are typically applied on adjacent pavements (curbs) to assist the bonding of the HMA and concrete.
Portland Cement Concrete (PCC) Layers
Portland cement concrete surface courses are referred to as rigid pavements, or concrete pavements. There are three general classifications of concrete pavements - jointed plain, jointed reinforced, and continuously reinforced.
Traffic loadings are transferred between sections when larger aggregates in the PCC mix inter-lock together, or through load transfer devices in the transverse joints of the surface. Dowel bars are used as load-transferring devices to efficiently transfer loads across transverse joints while maintaining the joint's horizontal and vertical alignment. Tie-bars are deformed steel bars that are placed along longitudinal joints to hold adjacent pavement sections in place.
The overall purpose of highway maintenance is to fix defects and preserve the pavement's structure and serviceability. Defects must be defined, understood, and recorded in order to select an appropriate maintenance plan. Defects differ between flexible and rigid pavements.
There are four main objectives of highway maintenance:
repair of functional pavement defects
extend the functional and structural service life of the pavement
maintain road safety and signage
keep road reserve in acceptable condition
Through routine maintenance practices, highway systems and all of their components can be maintained to their original, as-built condition.
Project management involves the organization and structuring of project activities from start to completion. Activities could be the construction of infrastructure such as highways and bridges or major and minor maintenance activities related to constructing such infrastructure. The entire project and involved activities must be handled in a professional manner and completed within deadlines and budget. In addition, minimizing social and environmental impacts is essential to successful project management.
Traditional track structure
Section through railway track and foundation showing the ballast and formation layers. The layers are slightly sloped to help drainage.
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 tie-plate 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. 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 dog-spikes through a flat tie-plate 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.
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.
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.
Ballast-less high-speed track in China
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 leads to heavy maintenance costs. This can be overcome by using ballast-less 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 a continuous 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 ballast-less 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 (e.g. rail station), or for localised replacement where there are exceptional maintenance difficulties, for example in tunnels.
Ladder track at Akabane Station, Kita, Tokyo, Japan
Ladder track utilizes sleepers aligned along the same direction as the rails with rung-like gauge restraining cross members. Both ballasted and ballastless types exist.
Diagram of cross section of 1830s ladder type track used on the Leeds and Selby Railway
Continuous longitudinally supported track
Early railways (c.1840s) experimented with continuous bearing rail track, in which the rail was supported along its length, with examples including Brunel's Baulk road on the Great Western Railway, as well as use on the Newcastle and North Shields Railway, on the Lancashire and Yorkshire Railway to a design by John Hawkshaw, and elsewhere. Continuous bearing designs were also promoted by other engineers. The system was trialed on the Baltimore and Ohio railway in the 1840s, but was found to be more expensive to maintain than rail with cross ties.
Modern applications of continuously supported track include Balfour Beatty's 'Embedded Slab Track' which uses a rounded rectangular rail profile (BB14072) embedded in a slip formed (or pre-cast) concrete base (development 2000s), the 'Embedded Rail Structure', used in the Netherlands since 1976, initially used a conventional UIC 54 rail embedded in concrete, later developed (late 1990s) to use a 'mushroom' shaped SA42 rail profile; a version for light rail using a rail supported in an asphalt concrete filled steel trough has also been developed (2002).
The technology of rail tracks developed over a long period, starting with primitive timber rails in mines in the 17th century.
Cross-sections of flat-bottomed rail, which can rest directly on the sleepers, and bullhead rail which sits in a chair. (not shown)
Modern track typically uses Hot rolled steel with a profile of an asymmetrical rounded I-beam. 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 track work the heavier and faster the trains the track can carry.
Rail from 1896 showing manufacturer's name and specification formed onto the web of rail during rolling
Other profiles of rail include: Bullhead rail; Grooved rail; "Flat-bottomed rail" (Vignoles rail or Flanged T rail; Bridge rail (inverted U shaped used in Baulk road; Barlow rail
Cross section of new flat bottomed rail
A baulk road crossing showing the baulks (under the rails) and transoms (to maintain the gauge)
Cross section of Barlow rail as used by Sydney Railway Company
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 (12.2 m) long; as gondola sizes increased, so did rail lengths.
A railroad gondola seen at Rochelle, Illinois
According to the Railway Gazette the 150 kilometer rail line being built for the Baffin land Iron Mine, on Baffin Island, will use older carbon steel alloys for its rails, instead of more modern, higher performance alloys, because modern alloy rails can become brittle at very low temperatures.
The earliest rails were made of wood, but these wore out quickly. Hardwood such as Jarrah and Karri were better than softwoods such as Fir. Longitudinal sleepers such as Brunel's baulk road are topped with iron or steel rails that are lighter than they then might otherwise be because of the support of the sleepers.
JarrahTree Karri Tree
Rail classification (weight)
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 Europe, rail is graded in kg/m and the usual range is 40 to 60 kg/m (80.6 to 121.0 lb/yd). 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). 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.
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).
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 ft (12 or 24 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 (2 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.
Fishplate on the Bluebell Railway (a heritage line running for 11 mi (17.7 km) along the border between East and West Sussex)
An expansion joint on the Cornish Main Line
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 breaking of the rail head (the running surface). This was the cause of the Hither Green rail crash near Hither Green maintenance depot, between Hither Green and Grove Park railway stations on 5 November 1967which caused British Railways to begin converting much of its track to Continuous Welded Rail.
Where track circuits exist for signaling 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.
Illustration of track circuit invented by William Robinson in 1872
As an alternative to the insulated joint, audio frequency track circuits can be employed using a tuned loop formed in approximately 20 m (66 ft) 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.
LC circuit diagram also called a resonant circuit, tank circuit, or tuned circuit
An axle counter detection point in the UK
Continuous welded rail
Welded rail joint
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 kilometers 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 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 unjointed 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 restraints are insufficient, the track could become distorted in hot weather and cause a derailment. Distortion due to heat expansion is known as sun kink or 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.
Sun kink in rail tracks
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 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.
Examples of buckling track in continuous welded rail (CWR)
Continuous welded rail CWR 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.
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.
A breather switch on a TGV line
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 (rail or track gauge is a technical term used in rail transport to define the spacing of the rails on a railway track and is measured between the inner faces of the load-bearing rails).They are generally laid transverse (perpendicular) to the rails.
Good quality track ballast is made of crushed stone. The sharp edges help the particles interlock with each other
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 (Pandrol is a British company that manufactures rail fastenings, which are used to fasten rails to railway sleepers).
A Pandrol E clip in use
Panama Canal construction track
Sometimes rail tracks are designed to be portable and moved from one place to another as required. During construction of the Panama Canal, tracks were moved around excavation works. These tracks were 5 ft (1,524 mm) and the rolling stock full size. Portable tracks have often been used in open pit mines.
Cane railways often had permanent tracks for the main lines, with portable tracks serving the cane fields 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.
Decauville a French manufacturing company was a source of many portable light rail tracks, also used for military purposes.
The geometry of the tracks is three-dimensional by nature, but the standards that express the speed limits and other regulations in the areas of track gauge, alignment, elevation, curvature and track surface are usually expressed in two separate layouts for horizontal and vertical.
Horizontal layout is the track layout on the horizontal plane. This involves the layout of three main track types: tangent track (straight line), curved track, and track transition curve (also called transition spiral or spiral) which connects between a tangent and a curved track.
Vertical layout is the track layout on the vertical plane including the concepts such as crosslevel, cant and gradient.
Main articles: Track gauge and List of track gauges
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 1⁄2 in), known as standard or international gauge. 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.
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 1⁄2 in (1,460 mm) for operation up to 60 mph (96.6 km/h).
USA section gang (gandy dancers) responsible for maintenance of a particular section of railway. One man is holding a lining bar (gandy), while others are using rail tongs to position a rail. Photo 1917
Further information: Rail inspection and Work train
See also Non-revenue cars
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, or trackmen (US: gandy dancers; UK: platelayers; 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.
Flange oilers lubricate wheel flanges to reduce rail wear in tight curves, Middelburg, Mpumalanga, South Africa
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. The process of crosstie and rail replacement can be automated by using a track renewal train.
Spraying ballast with herbicide to prevent weeds growing through and disrupting the ballast is typically done with a special weed killing train.
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.
Maintenance of way equipment in Italy
A tie replacement train in Pennsylvania
Rails must be replaced before the railhead profile wears to a degree that may trigger a derailment. Worn mainline rails usually have sufficient life remaining 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
On this Japanese high-speed line, mats have been added to stabilize the ballast
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. There 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.
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.