TGV track construction
Encyclopedia
LGV construction is the process by which the land on which TGV
trains are to run is prepared for their use, involving carving the trackbed and laying the track
. It is similar to the building of standard railway lines, but there are differences. In particular, construction process is more precise in order for the track to be suitable for regular use at 300 km/h (186 mph). The quality of construction was put to the test in particular during the TGV world speed record
runs on the LGV Atlantique
; the track was used at over 500 km/h (310 mph) without suffering significant damage. This contrasts with previous French world rail speed record attempts which resulted in severe deformation of the track.
) begins with earth moving. The trackbed is carved into the landscape, using scraper
s, grader
s, bulldozer
s and other heavy machinery
. All fixed structures are built; these include bridge
s, flyovers
, culvert
s, game tunnel
s, and the like. Drainage
facilities, most notably the large ditch
es on either side of the trackbed, are constructed. Supply bases are established near the end of the high-speed tracks, where crews will form work trains to carry rail, sleeper
s and other supplies to the work site.
Next, a layer of compact gravel
is spread on the trackbed. This, after being compacted by roller
s, provides an adequate surface for vehicles with tyre
s. TGV tracklaying then proceeds. The tracklaying process is not particularly specialized to high-speed lines; the same general technique is applicable to any track that uses continuous welded rail. The steps outlined below are used around the world in modern tracklaying. TGV track, however, answers to stringent requirements that dictate materials, dimension
s and tolerance
s.
that rides on rubber
tires is used to lay down panels of prefabricated
track. These are laid roughly in the location where one of the tracks will be built (all LGVs have two tracks). Each panel is 18 metres (60 feet
) long, and rests on wood
en sleeper
s. No ballast
is used at this stage, since the panel track is temporary.
Once the panel track is laid, a work train (pulled by diesel locomotive
s) can bring in the sections of continuous welded rail that will be used for the permanent way of this first track. The rail comes from the factory in lengths varying from 200 m (660 ft) to 400 m (1310 ft). Such long pieces of rail are just laid across several flatcar
s; they are very flexible, so this does not pose a problem. A special crane
unloads the rail sections and places them on each side of the temporary track, approximately 3.5 m (12 ft) apart. This operation is usually carried out at night, for thermal reasons. The rail itself is standard UIC
section, 60 kg/m (40 lb
/ft), with a tensile strength
of 800 newtons per square millimetre or megapascals (116,000 psi).
For the next step, a gantry crane is used again. This time, however, the crane rides on the two rails that were just laid alongside the temporary track. A train of flatcars, half loaded with LGV sleepers, arrives at the site. It is pushed by a special diesel locomotive, which is low enough to fit underneath the gantry cranes. The cranes remove the panels of temporary track, and stack them onto the empty half of the sleeper train. Next, they pick up sets of 30 LGV sleepers, pre-arranged with the proper spacing (60 cm, or 24 in), using a special fixture. The sleepers are laid on the gravel bed where the panel track was. The sleeper train leaves the worksite loaded with sections of panel track.
The sleepers, sometimes known as bi-bloc sleepers, are U41 twin block reinforced concrete
, 2.4 m (7 ft 10 in.) wide, and weigh 245 kg (540 lb) each. They are equipped with hardware for Nabla RNTC spring fasteners, and a 9 mm (3/8 in.) rubber pad. (Rubber pads are always used under the rail on concrete
sleepers, to avoid cracking). Next, a rail threader is used to lift the rails onto their final position on the sleepers. This machine rides on the rails just like the gantry cranes, but can also support itself directly on a sleeper. By doing this, it can lift the rails, and shift them inwards over the ends of the sleepers, to the proper gauge
(standard gauge
). It then lowers them onto the rubber sleeper cushions, and workers use a pneumatically operated
machine to bolt down the Nabla clips with a predetermined torque
. The rails are canted inward at a slope of 1 in 20.
. Conventional welding
(using some type of flame) does not work well on large metal
pieces such as rails, since the heat is conducted away
too quickly. Thermite is better suited to this job. It is a mix of aluminium
powder and rust
(iron oxide) powder, which reacts to produce iron
, aluminum oxide, and a great deal of heat, making it ideal to weld rail.
Before the rail is joined, its length must be adjusted very accurately. This ensures that the thermal stress
es in the rail after it is joined into one continuous piece do not exceed certain limits, resulting in lateral kink
s (in hot weather) or fracture
s (in cold weather). The joining operation is performed by an aluminothermic welding machine which is equipped with a rail saw
, a weld shear and a grinder. When the thermite welding process is complete, the weld is ground to the profile of the rail, resulting in a seamless join between rail sections. Stress in the rail due to temperature variations is absorbed without longitudinal strain
, except near bridges where an expansion joint
is sometimes used.
underneath the new track. The ballast arrives in a train of hopper car
s pulled by diesel locomotives. Handling this train is challenging, since the ballast must be spread evenly. If the train stops, ballast can pile over the rails and derail
it.
A first layer of ballast is dumped directly onto the track, and a tamping-lining-levelling machine
, riding on the rails, forces the stones underneath the sleepers. Each pass of this machine can raise the level of the track by 8 cm (3 in), so several passes of ballasting and of the machine are needed to build up a layer of ballast at least 32 cm (1 ft) thick under the sleepers. The ballast is also piled on each side of the track for lateral stability. The machine performs the initial alignment of the track. Next, a ballast regulator
distributes the ballast evenly. Finally, a dynamic vibrator
machine shakes the track to perform the final tamping, effectively simulating the passing of 2500 axle
s.
The two tracks are now essentially complete, but the work on the line is not finished. The catenary masts need to be erected, and the wire strung on them. Catenary installation is not complicated; it will suffice to give a brief summary of specifications. The steel masts are I-beam
s, placed in a concrete foundation up to 63 m (206 ft) apart. The supports are mounted on glass
insulator
s. The carrier wire is bronze
, 65 mm² cross section
, 14 kN (3100 lbf) tension. The stitch wire is bronze, 15 m (49.21 ft) long, 35 mm² cross-section. The droppers are 5 mm stranded copper
cable. The contact wire is hard drawn copper, 120 mm², flat section on the contact side, 14 kN tension. The maximum depth of the catenary (distance between carrier and contact wires) is 1.4 m (4.59 ft). The contact wire can rise a maximum of 240 mm (9.44 inches) but the normal vertical displacement does not exceed 120 mm (4.72 inches).
Now that the catenary is complete, the track is given final alignment adjustments down to millimeter tolerances. The ballast is then blown to remove smaller gravel fragments and dust, which might be kicked up by trains. This step is especially important on high-speed tracks, since the blast of a passing train is strong. Finally, TGV
trains are tested on the line at gradually increasing speeds. The track is qualified at speeds slightly higher than will be used in everyday operations (typically 350 km/h, or 210 mph), before being opened to commercial service.
TGV
The TGV is France's high-speed rail service, currently operated by SNCF Voyages, the long-distance rail branch of SNCF, the French national rail operator....
trains are to run is prepared for their use, involving carving the trackbed and laying the track
Rail tracks
The track on a railway or railroad, also known as the permanent way, is the structure consisting of the rails, fasteners, sleepers and ballast , plus the underlying subgrade...
. It is similar to the building of standard railway lines, but there are differences. In particular, construction process is more precise in order for the track to be suitable for regular use at 300 km/h (186 mph). The quality of construction was put to the test in particular during the TGV world speed record
TGV world speed record
The TGV holds a series of world speed records achieved by SNCF, the French national railway, and its industrial partners...
runs on the LGV Atlantique
LGV Atlantique
The LGV Atlantique is a high-speed railway line running from Paris to Western France. It opened in 1989-1990. It divides into two parts at Courtalain, one going westward to Le Mans , the second one going southwestward to Tours .-Route:The line leaves Gare Montparnasse to cross Paris's southern...
; the track was used at over 500 km/h (310 mph) without suffering significant damage. This contrasts with previous French world rail speed record attempts which resulted in severe deformation of the track.
Preparing the trackbed
The work on a high-speed line (ligne à grande vitesse, or LGVLGV
LGV may stand for:* Large Goods Vehicle in Europe.* Lymphogranuloma venereum* Lignes à Grande Vitesse, French high-speed rail lines:** LGV Atlantique** LGV Est** LGV Interconnexion Est** LGV Nord** LGV Méditerranée** LGV Picardie** LGV Rhône-Alpes...
) begins with earth moving. The trackbed is carved into the landscape, using scraper
Wheel tractor-scraper
In civil engineering, a wheel tractor-scraper is a piece of heavy equipment used for earthmoving.The rear part has a vertically moveable hopper with a sharp horizontal front edge. The hopper can be hydraulically lowered and raised. When the hopper is lowered, the front edge cuts into the soil or...
s, grader
Grader
A grader, also commonly referred to as a road grader, a blade, a maintainer, or a motor grader, is a construction machine with a long blade used to create a flat surface. Typical models have three axles, with the engine and cab situated above the rear axles at one end of the vehicle and a third...
s, bulldozer
Bulldozer
A bulldozer is a crawler equipped with a substantial metal plate used to push large quantities of soil, sand, rubble, etc., during construction work and typically equipped at the rear with a claw-like device to loosen densely-compacted materials.Bulldozers can be found on a wide range of sites,...
s and other heavy machinery
Engineering vehicle
Heavy equipment refers to heavy-duty vehicles, specially designed for executing construction tasks, most frequently ones involving earthwork operations. They are also known as, construction equipment, construction plant, earth movers, engineering vehicles, or simply equipment...
. All fixed structures are built; these include bridge
Bridge
A bridge is a structure built to span physical obstacles such as a body of water, valley, or road, for the purpose of providing passage over the obstacle...
s, flyovers
Overpass
An overpass is a bridge, road, railway or similar structure that crosses over another road or railway...
, culvert
Culvert
A culvert is a device used to channel water. It may be used to allow water to pass underneath a road, railway, or embankment. Culverts can be made of many different materials; steel, polyvinyl chloride and concrete are the most common...
s, game tunnel
Wildlife crossing
Wildlife crossings are structures that allow animals to cross human-made barriers safely. Wildlife crossings may include: underpass tunnels, viaducts, and overpasses ; amphibian tunnels; fish ladders; tunnels and culverts ; green roofs .Wildlife crossings are a...
s, and the like. Drainage
Drainage
Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...
facilities, most notably the large ditch
Ditch
A ditch is usually defined as a small to moderate depression created to channel water.In Anglo-Saxon, the word dïc already existed and was pronounced 'deek' in northern England and 'deetch' in the south. The origins of the word lie in digging a trench and forming the upcast soil into a bank...
es on either side of the trackbed, are constructed. Supply bases are established near the end of the high-speed tracks, where crews will form work trains to carry rail, sleeper
Railroad tie
A railroad tie/railway tie , or railway sleeper is a rectangular item used to support the rails in railroad tracks...
s and other supplies to the work site.
Next, a layer of compact gravel
Gravel
Gravel is composed of unconsolidated rock fragments that have a general particle size range and include size classes from granule- to boulder-sized fragments. Gravel can be sub-categorized into granule and cobble...
is spread on the trackbed. This, after being compacted by roller
Road roller
A road roller is a compactor type engineering vehicle used to compact soil, gravel, concrete, or asphalt in the construction of roads and foundations, similar rollers are used also at landfills or in agriculture.In some parts of the world, road rollers are still known colloquially as steam...
s, provides an adequate surface for vehicles with tyre
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
s. TGV tracklaying then proceeds. The tracklaying process is not particularly specialized to high-speed lines; the same general technique is applicable to any track that uses continuous welded rail. The steps outlined below are used around the world in modern tracklaying. TGV track, however, answers to stringent requirements that dictate materials, dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...
s and tolerance
Tolerance (engineering)
Engineering tolerance is the permissible limit or limits of variation in# a physical dimension,# a measured value or physical property of a material, manufactured object, system, or service,# other measured values ....
s.
Laying the track
To begin laying track, a gantry cranePortainer
A container crane is a type of large dockside gantry crane found at container terminals for loading and unloading intermodal containers from container ships....
that rides on rubber
Rubber
Natural rubber, also called India rubber or caoutchouc, is an elastomer that was originally derived from latex, a milky colloid produced by some plants. The plants would be ‘tapped’, that is, an incision made into the bark of the tree and the sticky, milk colored latex sap collected and refined...
tires is used to lay down panels of prefabricated
Prefabrication
Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting complete assemblies or sub-assemblies to the construction site where the structure is to be located...
track. These are laid roughly in the location where one of the tracks will be built (all LGVs have two tracks). Each panel is 18 metres (60 feet
Foot (unit of length)
A foot A foot A foot (plural: feet; abbreviation or symbol: ft or ′ (the prime symbol) is a unit of length in a number of different systems including Ancient Greek, Ancient Roman, English units, Imperial units, United States customary units and the units of many Continental European...
) long, and rests on wood
Wood
Wood is a hard, fibrous tissue found in many trees. It has been used for hundreds of thousands of years for both fuel and as a construction material. It is an organic material, a natural composite of cellulose fibers embedded in a matrix of lignin which resists compression...
en sleeper
Railroad tie
A railroad tie/railway tie , or railway sleeper is a rectangular item used to support the rails in railroad tracks...
s. No ballast
Track ballast
Track ballast forms the trackbed upon which railway sleepers or railroad ties are laid. It is packed between, below, and around the ties. It is used to facilitate drainage of water, to distribute the load from the railroad ties, and also to keep down vegetation that might interfere with the track...
is used at this stage, since the panel track is temporary.
Once the panel track is laid, a work train (pulled by diesel locomotive
Diesel locomotive
A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...
s) can bring in the sections of continuous welded rail that will be used for the permanent way of this first track. The rail comes from the factory in lengths varying from 200 m (660 ft) to 400 m (1310 ft). Such long pieces of rail are just laid across several flatcar
Flatcar
A flatcar is a piece of railroad or railway rolling stock that consists of an open, flat deck on four or six wheels or a pair of trucks or bogies . The deck of the car can be wood or steel, and the sides of the deck can include pockets for stakes or tie-down points to secure loads...
s; they are very flexible, so this does not pose a problem. A special crane
Crane (machine)
A crane is a type of machine, generally equipped with a hoist, wire ropes or chains, and sheaves, that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of...
unloads the rail sections and places them on each side of the temporary track, approximately 3.5 m (12 ft) apart. This operation is usually carried out at night, for thermal reasons. The rail itself is standard UIC
International Union of Railways
The UIC , or International Union of Railways, is an international rail transport industry body.- Brief history :The railways of Europe originated as separate concerns. There were many border changes after World War I and the Treaty of Versailles. Colonial railways were the responsibility of the...
section, 60 kg/m (40 lb
Pound (mass)
The pound or pound-mass is a unit of mass used in the Imperial, United States customary and other systems of measurement...
/ft), with a tensile strength
Tensile strength
Ultimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
of 800 newtons per square millimetre or megapascals (116,000 psi).
For the next step, a gantry crane is used again. This time, however, the crane rides on the two rails that were just laid alongside the temporary track. A train of flatcars, half loaded with LGV sleepers, arrives at the site. It is pushed by a special diesel locomotive, which is low enough to fit underneath the gantry cranes. The cranes remove the panels of temporary track, and stack them onto the empty half of the sleeper train. Next, they pick up sets of 30 LGV sleepers, pre-arranged with the proper spacing (60 cm, or 24 in), using a special fixture. The sleepers are laid on the gravel bed where the panel track was. The sleeper train leaves the worksite loaded with sections of panel track.
The sleepers, sometimes known as bi-bloc sleepers, are U41 twin block reinforced concrete
Reinforced concrete
Reinforced concrete is concrete in which reinforcement bars , reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is...
, 2.4 m (7 ft 10 in.) wide, and weigh 245 kg (540 lb) each. They are equipped with hardware for Nabla RNTC spring fasteners, and a 9 mm (3/8 in.) rubber pad. (Rubber pads are always used under the rail on concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
sleepers, to avoid cracking). Next, a rail threader is used to lift the rails onto their final position on the sleepers. This machine rides on the rails just like the gantry cranes, but can also support itself directly on a sleeper. By doing this, it can lift the rails, and shift them inwards over the ends of the sleepers, to the proper gauge
Rail gauge
Track gauge or rail gauge is the distance between the inner sides of the heads of the two load bearing rails that make up a single railway line. Sixty percent of the world's railways use a standard gauge of . Wider gauges are called broad gauge; smaller gauges, narrow gauge. Break-of-gauge refers...
(standard gauge
Standard gauge
The standard gauge is a widely-used track gauge . Approximately 60% of the world's existing railway lines are built to this gauge...
). It then lowers them onto the rubber sleeper cushions, and workers use a pneumatically operated
Pneumatics
Pneumatics is a branch of technology, which deals with the study and application of use of pressurized gas to effect mechanical motion.Pneumatic systems are extensively used in industry, where factories are commonly plumbed with compressed air or compressed inert gases...
machine to bolt down the Nabla clips with a predetermined torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
. The rails are canted inward at a slope of 1 in 20.
Joining track sections
The sections of rail are welded together using thermiteThermite
Thermite is a pyrotechnic composition of a metal powder and a metal oxide that produces an exothermic oxidation-reduction reaction known as a thermite reaction. If aluminium is the reducing agent it is called an aluminothermic reaction...
. Conventional welding
Welding
Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, with pressure sometimes...
(using some type of flame) does not work well on large metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
pieces such as rails, since the heat is conducted away
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
too quickly. Thermite is better suited to this job. It is a mix of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
powder and rust
Rust
Rust is a general term for a series of iron oxides. In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture...
(iron oxide) powder, which reacts to produce iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, aluminum oxide, and a great deal of heat, making it ideal to weld rail.
Before the rail is joined, its length must be adjusted very accurately. This ensures that the thermal stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
es in the rail after it is joined into one continuous piece do not exceed certain limits, resulting in lateral kink
Buckling
In science, buckling is a mathematical instability, leading to a failure mode.Theoretically, buckling is caused by a bifurcation in the solution to the equations of static equilibrium...
s (in hot weather) or fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
s (in cold weather). The joining operation is performed by an aluminothermic welding machine which is equipped with a rail saw
Saw
A saw is a tool that uses a hard blade or wire with an abrasive edge to cut through softer materials. The cutting edge of a saw is either a serrated blade or an abrasive...
, a weld shear and a grinder. When the thermite welding process is complete, the weld is ground to the profile of the rail, resulting in a seamless join between rail sections. Stress in the rail due to temperature variations is absorbed without longitudinal strain
Strain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
, except near bridges where an expansion joint
Expansion joint
An expansion joint or movement joint is an assembly designed to safely absorb the heat-induced expansion and contraction of various construction materials, to absorb vibration, to hold certain parts together, or to allow movement due to ground settlement or earthquakes...
is sometimes used.
Adding ballast
The next step consists of stuffing a deep bed of ballastTrack ballast
Track ballast forms the trackbed upon which railway sleepers or railroad ties are laid. It is packed between, below, and around the ties. It is used to facilitate drainage of water, to distribute the load from the railroad ties, and also to keep down vegetation that might interfere with the track...
underneath the new track. The ballast arrives in a train of hopper car
Hopper car
A hopper car is a type of railroad freight car used to transport loose bulk commodities such as coal, ore, grain, track ballast, and the like. The name originated from the coke manufacturing industry which is part of the steel industry ....
s pulled by diesel locomotives. Handling this train is challenging, since the ballast must be spread evenly. If the train stops, ballast can pile over the rails and derail
Derailment
A derailment is an accident on a railway or tramway in which a rail vehicle, or part or all of a train, leaves the tracks on which it is travelling, with consequent damage and in many cases injury and/or death....
it.
A first layer of ballast is dumped directly onto the track, and a tamping-lining-levelling machine
Ballast tamper
A ballast tamper or tamping machine is a machine used to pack the track ballast under railway tracks to make the tracks more durable. Prior to the introduction of mechanical tampers, this task was done by manual labour with the help of beaters...
, riding on the rails, forces the stones underneath the sleepers. Each pass of this machine can raise the level of the track by 8 cm (3 in), so several passes of ballasting and of the machine are needed to build up a layer of ballast at least 32 cm (1 ft) thick under the sleepers. The ballast is also piled on each side of the track for lateral stability. The machine performs the initial alignment of the track. Next, a ballast regulator
Ballast regulator
A ballast regulator is a piece of rail transport maintenance of way equipment used to shape and distribute the gravel track ballast that supports the ties in rail tracks. They are often used in conjunction with ballast tampers when maintaining track....
distributes the ballast evenly. Finally, a dynamic vibrator
Vibrator (mechanical)
A vibrator is a mechanical device to generate vibrations. The vibration is often generated by an electric motor with an unbalanced mass on its driveshaft.There are many different types of vibrator...
machine shakes the track to perform the final tamping, effectively simulating the passing of 2500 axle
Axle
An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...
s.
Finishing construction
Now that the first track is almost complete, work begins on the adjacent track. This time, however, it is not necessary to lay a temporary track. Trains running on the first track bring the sleepers, and then the rail, which is unloaded directly onto the sleepers by dispensing arms that swing out to the proper alignment. The Nabla fasteners are secured, and the ballast is stuffed under the track as before.The two tracks are now essentially complete, but the work on the line is not finished. The catenary masts need to be erected, and the wire strung on them. Catenary installation is not complicated; it will suffice to give a brief summary of specifications. The steel masts are I-beam
I-beam
-beams, also known as H-beams, W-beams , rolled steel joist , or double-T are beams with an - or H-shaped cross-section. The horizontal elements of the "" are flanges, while the vertical element is the web...
s, placed in a concrete foundation up to 63 m (206 ft) apart. The supports are mounted on glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
insulator
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
s. The carrier wire is bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...
, 65 mm² cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
, 14 kN (3100 lbf) tension. The stitch wire is bronze, 15 m (49.21 ft) long, 35 mm² cross-section. The droppers are 5 mm stranded copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
cable. The contact wire is hard drawn copper, 120 mm², flat section on the contact side, 14 kN tension. The maximum depth of the catenary (distance between carrier and contact wires) is 1.4 m (4.59 ft). The contact wire can rise a maximum of 240 mm (9.44 inches) but the normal vertical displacement does not exceed 120 mm (4.72 inches).
Now that the catenary is complete, the track is given final alignment adjustments down to millimeter tolerances. The ballast is then blown to remove smaller gravel fragments and dust, which might be kicked up by trains. This step is especially important on high-speed tracks, since the blast of a passing train is strong. Finally, TGV
TGV
The TGV is France's high-speed rail service, currently operated by SNCF Voyages, the long-distance rail branch of SNCF, the French national rail operator....
trains are tested on the line at gradually increasing speeds. The track is qualified at speeds slightly higher than will be used in everyday operations (typically 350 km/h, or 210 mph), before being opened to commercial service.