|LGV Track Construction|
The work on a high speed line (ligne à grande vitesse, or LGV) begins with earth moving. The trackbed is carved into the landscape, using scrapers, graders, bulldozers and other heavy machinery. All fixed structures are built; these include bridges, overpasses, culverts, game tunnels, and the like. Drainage facilities, most notably the large ditches 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, ties and other supplies to the work site.
Next, a layer of compact gravel is spread on the trackbed. This, after being compacted by rollers, provides an adequate surface for vehicles with tires. TGV track laying then proceeds. The track laying 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 track laying. TGV track, however, answers to stringent requirements that dictate materials, dimensions and tolerances.
To begin laying track, a gantry crane 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 are double tracked). Each panel is 18 meters (60 ft.) long, and rests on wooden cross ties. 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 locomotives) can bring in the sections of continuous welded rail that will be used for this first track (of two). 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 flat cars; 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 meters (12 feet) 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 millimeter (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 flat cars, half loaded with TGV cross ties, 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 tie train. Next, they pick up sets of 30 TGV ties, pre-arranged with the proper spacing (60 cm, or 24"), using a special fixture. The ties are laid on the gravel bed where the panel track used to be. The tie train leaves the worksite loaded with sections of panel track.
The tie train with gantry cranes at work. Photo: C. Recoura / LVDR
Ties being placed on the railroad bed. Photo: Seco-DG
The cross ties are U41 twin block reinforced concrete, 2.4 m (7' 10") wide, and weigh 245 kg (540 lb) each. They are equipped with hardware for Nabla RNTC spring fasteners, and a 9 mm (3/8") rubber pad. (Rubber pads are always used under the rail on concrete ties, to avoid cracking). Next, a rail threader is used to lift the rails onto their final position on the ties. This machine rides on the rails just like the gantry cranes, but can also support itself directly on a tie. By doing this, it can lift the rails, and shift them inwards over the ends of the ties, to the proper gauge. It then lowers them onto the rubber tie 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; this is clearly visible on photographs of the tie cushions.
Nabla fastener, made by Stedef.
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The rails are placed on the ties. Note how flexible they are. Photo: C. Recoura / LVDR
The sections of rail are welded together using thermite. Conventional welding (using some type of flame) does not work well on large metal pieces such as rail, since the heat is conducted away too quickly. Thermite is better suited to this job. It is a mix of aluminum powder and rust (iron oxide) powder, which reacts to produce iron, aluminum oxide, and a very large amount of heat. This last property is what makes it ideal to join pieces of rail. Before the rail is joined, its length must be adjusted very accurately. This ensures that the thermal stresses in the rail after it is joined into one continuous piece do not exceed certain limits, resulting in lateral kinks (in hot weather) or breaks (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 down to the profile of the rail, resulting in a seamless connection between adjoining sections. Thermal stresses in the rail due to varying ambient temperatures are absorbed without longitudinal strain, except near bridges where an expansion joint is sometimes used.
The next step consists of stuffing a deep bed of ballast underneath the new track. The ballast arrives in a train of hopper cars pulled by one or two diesels. Handling this train is challenging, since the ballast must be spread as evenly as possible. 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 ties. Each pass of this machine can raise the level of the track by 8 cm (3 inches), so several passes of ballasting and of the machine are needed to build up a layer of ballast at least 32 cm (1 foot) think under the ties. The ballast is also piled thick 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 axles.
The tamping machine.
Photo: C. Recoura / LVDR
The ballast regulator. Photo: C. Recoura / LVDR
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 ties, and then the rail, which is unloaded directly onto the ties 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-beams, placed in a concrete foundation up to 63 m (206 ft) apart. The supports are mounted on glass insulators. The carrier wire is bronze, 65 mm^2 cross section, 14 kN (3100 lb) tension. The stitch wire is bronze, 15 m (49 ft) long, 35 mm^2 cross section, The droppers are 5 mm stranded copper cable. The contact wire is hard drawn copper, 120 mm^2, 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 feet). The contact wire can rise a maximum of 240 mm (9 inches) but the normal vertical displacement does not exceed 120 mm (4 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 212 mph), before being opened to commercial service.
Last Update: February 2000